Major League Baseball (MLB) is a professional baseball organization and the oldest of the major professional sports leagues in the United States and Canada. A total of 30 teams play in the National League (NL) and American League (AL), with 15 teams in each league. The NL and AL were formed as separate legal entities in 1876 and 1901, respectively. After cooperating but remaining legally separate entities beginning in 1903, the leagues merged into a single organization led by the Commissioner of Baseball in 2000.^[8][9] The organization also oversees Minor League Baseball, which comprises 256 teams affiliated with the major league clubs. With the World Baseball Softball Confederation, MLB manages the international World Baseball Classic tournament.

Baseball's first openly all-professional team was founded in Cincinnati in 1869. (Previously, some teams had paid some players under the table.) The first few decades of professional baseball were characterized by rivalries between leagues and by players who often jumped from one team or league to another.

The period before 1920 in baseball is known as the dead-ball era, during which players would rarely hit home runs. In the late 1910s, baseball survived a conspiracy to fix the 1919 World Series, which came to be known as the Black Sox Scandal. The sport rose in popularity in the 1920s, and survived potential downturns during the Great Depression and World War II. Shortly after the war, Jackie Robinson broke baseball's color barrier.

The 1950s and 1960s were a time of expansion for the AL and NL, then new stadiums and artificial turf surfaces began to change the game in the 1970s and 1980s. Home runs dominated the game during the 1990s, and media reports began to discuss the use of anabolic steroids among MLB players in the mid-2000s. In 2006, an investigation produced the Mitchell Report, which implicated many players in the use of performance-enhancing substances, including at least one player from each team.

Today, MLB is composed of 30 teams: 29 in the United States and 1 in Canada. Teams play 162 games each season and five teams in each league advance to a four-round postseason tournament that culminates in the World Series, a best-of-seven championship series between the two league champions that dates to 1903. Baseball broadcasts are aired on television, radio, and the Internet throughout North America and in several other countries throughout the world. MLB has the highest season attendance of any sports league in the world with more than 69.6 million spectators in 2018.^[10]

MLB is governed by the Major League Baseball Constitution. This document has undergone several incarnations since its creation in 1876.^[11] Under the direction of the Commissioner of Baseball, MLB hires and maintains the sport's umpiring crews, and negotiates marketing, labor, and television contracts. MLB maintains a unique, controlling relationship over the sport, including most aspects of Minor League Baseball. This is due in large part to the 1922 U.S. Supreme Court ruling in Federal Baseball Club v. National League, which held that baseball is not interstate commerce and therefore not subject to federal antitrust law. This ruling has been weakened only slightly in subsequent years.^[12] The weakened ruling granted more stability to the owners of teams and has resulted in values increasing at double-digit rates.^[12][13] There were several challenges to MLB's primacy in the sport between the 1870s and the Federal League in 1916; the last attempt at a new major league was the aborted Continental League in 1960.^[12]

The chief executive of MLB is the commissioner, currently Rob Manfred. The chief operating officer is Tony Petitti. There are five other executives: president (business and media), chief communications officer, chief legal officer, chief financial officer, and chief baseball officer.^[14][15]

The multimedia branch of MLB, which is based in Manhattan, is MLB Advanced Media. This branch oversees MLB.com and each of the 30 teams' websites. Its charter states that MLB Advanced Media holds editorial independence from the league, but it is under the same ownership group and revenue-sharing plan. MLB Productions is a similarly structured wing of the league, focusing on video and traditional broadcast media. MLB also owns 67 percent of MLB Network, with the other 33 percent split between several cable operators and satellite provider DirecTV.^[16] It operates out of studios in Secaucus, New Jersey, and also has editorial independence from the league.^[17]

In 1920, the weak National Commission, which had been created to manage relationships between the two leagues, was replaced with the much more powerful Commissioner of Baseball, who had the power to make decisions for all of professional baseball unilaterally.^[18] From 1901 to 1960, the American and National Leagues fielded eight teams apiece.

In the 1960s, MLB expansion added eight teams, including the first non-U.S. team (the Montreal Expos). Two teams (the Seattle Mariners and the Toronto Blue Jays) were also added in the 1970s. From 1969 through 1993, each league consisted of an East and West Division. A third division, the Central Division, was formed in each league in 1994. Until 1996, the two leagues met on the field only during the World Series and the All-Star Game. Regular-season interleague play was introduced in 1997.^[19]

In March 1995 two new franchises, the Arizona Diamondbacks and Tampa Bay Devil Rays (now known as the Tampa Bay Rays), were awarded by MLB, to begin play in 1998. This addition brought the total number of franchises to 30. In early 1997, MLB decided to assign one new team to each league: Tampa Bay joined the AL and Arizona joined the NL. The original plan was to have an odd number of teams in each league (15 per league, with five in each division), but in order for every team to be able to play daily, this would have required interleague play to be scheduled throughout the entire season. However, it was unclear at the time if interleague play would continue after the 1998 season, as it had to be approved by the players' union. For this and other reasons, it was decided that both leagues should continue to have an even number of teams; one existing club would have to switch leagues. The Milwaukee Brewers agreed in November 1997 to move from the AL to the NL, thereby making the NL a 16-team league. At the same time, the Detroit Tigers agreed to move from the AL East to the AL Central (to replace Milwaukee), with the expansion Tampa Bay Devil Rays joining the AL East.^[20] Later, when the Houston Astros changed ownership prior to the 2013 season, the team moved from the NL Central to the AL West,^[21][22] resulting in both leagues having three divisions of five teams each and allowing all teams to have a more balanced schedule.^[21] Interleague play is now held throughout the season.^[21]

In 2000 the AL and NL were dissolved as legal entities, and MLB became a single, overall league de jure, similar to the National Football League (NFL), National Basketball Association (NBA) and National Hockey League (NHL)—albeit with two components called "leagues" instead of "conferences." The same rules and regulations are used in both leagues, with one exception: the AL operates under the designated hitter rule, while the NL does not.^[23][24] This difference in rules between leagues is unique to MLB; the other sports leagues of the U.S. and Canada have one set of rules for all teams.

An asterisk () denotes a relocation of a franchise. See respective team articles for more information.*

In the 1860s, aided by soldiers playing the game in camp during the Civil War, "New York"-style baseball expanded into a national game and spawned baseball's first governing body, The National Association of Base Ball Players. The NABBP existed as an amateur league for 12 years. By 1867, more than 400 clubs were members. Most of the strongest clubs remained those based in the northeastern U.S. For professional baseball's founding year, MLB uses the year 1869—when the first professional team, the Cincinnati Red Stockings, was established.^[55]

A schism developed between professional and amateur ballplayers after the founding of the Cincinnati club. The NABBP split into an amateur organization and a professional organization. The National Association of Professional Base Ball Players, often known as the National Association (NA), was formed in 1871.^[56] Its amateur counterpart disappeared after only a few years.^[57] The modern Chicago Cubs and Atlanta Braves franchises trace their histories back to the National Association of Professional Base Ball Players in the 1870s.^[58]

In 1876, the National League of Professional Base Ball Clubs (later known as the National League or NL) was established after the NA proved ineffective. The league placed its emphasis on clubs rather than on players. Clubs could now enforce player contracts, preventing players from jumping to higher-paying clubs. Clubs were required to play the full schedule of games instead of forfeiting scheduled games when the club was no longer in the running for the league championship, which happened frequently under the NA. A concerted effort was made to curb gambling on games, which was leaving the validity of results in doubt. The first game in the NL—on Saturday, April 22, 1876 (at the Jefferson Street Grounds, Philadelphia)—is often pointed to as the beginning of MLB.^[2][59]

The early years of the NL were tumultuous, with threats from rival leagues and a rebellion by players against the hated "reserve clause", which restricted the free movement of players between clubs. Competitor leagues formed regularly and also disbanded regularly. The most successful was the American Association (1882–1891), sometimes called the "beer and whiskey league" for its tolerance of the sale of alcoholic beverages to spectators. For several years, the NL and American Association champions met in a postseason championship series—the first attempt at a World Series. The two leagues merged in 1892 as a single 12-team NL, but the NL dropped four teams after the 1899 season. This led to the formation of the American League in 1901 under AL president Ban Johnson, and the resulting bidding war for players led to widespread contract-breaking and legal disputes.

The war between the AL and NL caused shock waves throughout the baseball world. At a meeting at the Leland Hotel in Chicago in 1901, the other baseball leagues negotiated a plan to maintain their independence. A new National Association was formed to oversee these minor leagues.^[60] While the NA continues to this day (known as Minor League Baseball), at the time Ban Johnson saw it as a tool to end threats from smaller rivals who might expand in other territories and threaten his league's dominance.

After 1902, the NL, AL, and NA signed a new National Agreement which tied independent contracts to the reserve-clause contracts. The agreement also set up a formal classification system for minor leagues, the forerunner of today's system that was refined by Branch Rickey.^[61]

Several other early defunct baseball leagues are officially considered major leagues, and their statistics and records are included with those of the two current major leagues. These include the AA, the Union Association (1884),^[62] the Players' League (1890),^[63] and the Federal League (1914–1915).^[64][65] Both the UA and AA are considered major leagues by many baseball researchers because of the perceived high caliber of play and the number of star players featured. Some researchers, including Nate Silver, dispute the major-league status of the UA by pointing out that franchises came and went and that the St. Louis club was deliberately "stacked"; the St. Louis club was owned by the league's president and it was the only club that was close to major-league caliber.^[66]

The period between 1900 and 1919 is commonly referred to as the "Dead-Ball Era." Games of this era tended to be low scoring and were often dominated by pitchers, such as Walter Johnson, Cy Young, Christy Mathewson, Mordecai Brown, and Grover Cleveland Alexander. The term also accurately describes the condition of the baseball itself. The baseball used American rather than the modern Australian wool yarn and was not wound as tightly, affecting the distance that it would travel.^[67] More significantly, balls were kept in play until they were mangled, soft and sometimes lopsided. During this era, a baseball cost three dollars, equal to $43.35 today (in inflation-adjusted U.S. dollars), and owners were reluctant to purchase new balls. Fans were expected to throw back fouls and (rare) home runs. Baseballs also became stained with tobacco juice, grass, and mud, and sometimes the juice of licorice, which some players would chew for the purpose of discoloring the ball.^[68]

Also, pitchers could manipulate the ball through the use of the spitball. (In 1921 use of this pitch was restricted to a few pitchers with a grandfather clause). Additionally, many ballparks had large dimensions, such as the West Side Grounds of the Chicago Cubs, which was 560 feet (170 m) to the center field fence, and the Huntington Avenue Grounds of the Boston Red Sox, which was 635 feet (194 m) to the center field fence, thus home runs were rare, and "small ball" tactics such as singles, bunts, stolen bases, and the hit-and-run play dominated the strategies of the time.^[69] Hitting methods like the Baltimore Chop were used to increase the number of infield singles.^[70] On a successful Baltimore chop, the batter hits the ball forcefully into the ground, causing it to bounce so high that the batter reaches first base before the ball can be fielded and thrown to the first baseman.^[71]

The adoption of the foul strike rule in the early twentieth century quickly sent baseball from a high-scoring game to one where scoring runs became a struggle. Prior to the institution of this rule, foul balls were not counted as strikes: a batter could foul off any number of pitches with no strikes counted against him; this gave an enormous advantage to the batter. In 1901, the NL adopted the foul strike rule, and the AL followed suit in 1903.^[72]

After the 1919 World Series between the Chicago White Sox and Cincinnati Reds, baseball was rocked by allegations of a game fixing scheme known as the Black Sox Scandal. Eight players—Joe Jackson, Eddie Cicotte, Claude "Lefty" Williams, George "Buck" Weaver, Arnold "Chick" Gandil, Fred McMullin, Charles "Swede" Risberg, and Oscar "Happy" Felsch—intentionally lost the World Series in exchange for a ring worth $100,000 ($1,064,705.88 in 2016 dollars).^[73] Despite being acquitted, all were permanently banned from Major League Baseball.^[74]

Baseball's popularity increased in the 1920s and 1930s. The 1920 season was notable for the death of Ray Chapman of the Cleveland Indians. Chapman, who was struck in the head by a pitch and died a few hours later, became the only MLB player to die of an on-field injury, a tragedy which led directly to both leagues requiring the placing into play new, white baseballs whenever a ball became scuffed or dirty, helping bring the "dead-ball" era to an end. The following year, the New York Yankees made their first World Series appearance.^[75] By the end of the 1930s, the team had appeared in 11 World Series, winning eight of them.^[76] Yankees slugger Babe Ruth had set the single season home run record in 1927, hitting 60 home runs; a few years earlier, Ruth had set the same record with 29 home runs.^[77]

Affected by the difficulties of the Great Depression, baseball's popularity had begun a downward turn in the early 1930s. By 1932, only two MLB teams turned a profit. Attendance had fallen, due at least in part to a 10% federal amusement tax added to baseball ticket prices. Baseball owners cut their rosters from 25 men to 23 men, and even the best players took pay cuts. Team executives were innovative in their attempts to survive, creating night games, broadcasting games live by radio and rolling out promotions such as free admission for women. Throughout the period of the Great Depression, no MLB teams moved or folded.^[78]

The onset of World War II created a significant shortage of professional baseball players, as more than 500 men left MLB teams to serve in the military. Many of them played on service baseball teams that entertained military personnel in the US or in the Pacific. MLB teams of this time largely consisted of young men, older players, and those with a military classification of 4F, indicating mental, physical, or moral unsuitability for service. Men like Pete Gray, a one-armed outfielder, got the chance to advance to the major leagues. However, MLB rosters did not include any black players through the end of the war.^[79] Black players, many of whom served in the war, were still restricted to playing Negro league baseball.^[80]

Wartime blackout restrictions, designed to keep outdoor lighting at low levels, caused another problem for baseball. These rules limited traveling and night games to the point that the 1942 season nearly had to be canceled.^[80] On January 14, 1942, MLB Commissioner Kenesaw Mountain Landis wrote a letter to U.S. President Franklin D. Roosevelt and pleaded for the continuation of baseball during the war in hopes for a start of a new major league season. President Roosevelt responded, "I honestly feel that it would be best for the country to keep baseball going. There will be fewer people unemployed and everybody will work longer hours and harder than ever before. And that means that they ought to have a chance for recreation and for taking their minds off their work even more than before."^[81]

With the approval of President Roosevelt, spring training began in 1942 with few repercussions. The war interrupted the careers of stars including Stan Musial, Bob Feller, Ted Williams, and Joe DiMaggio, but baseball clubs continued to field their teams.^[82]

Branch Rickey, president and general manager of the Brooklyn Dodgers, began making efforts to introduce a black baseball player to the previously all-white professional baseball leagues in the mid-1940s. He selected Jackie Robinson from a list of promising Negro league players. After obtaining a commitment from Robinson to "turn the other cheek" to any racial antagonism directed at him, Rickey agreed to sign him to a contract for $600 a month. In what was later referred to as "The Noble Experiment", Robinson was the first black baseball player in the International League since the 1880s, joining the Dodgers' farm club, the Montreal Royals, for the 1946 season.^[83]

The following year, the Dodgers called up Robinson to the major leagues. On April 15, 1947, Robinson made his major league debut at Ebbets Field before a crowd of 26,623 spectators, including more than 14,000 black patrons. Black baseball fans began flocking to see the Dodgers when they came to town, abandoning the Negro league teams that they had followed exclusively. Robinson's promotion met a generally positive, although mixed, reception among newspaper writers and white major league players. Manager Leo Durocher informed his team, "I do not care if the guy is yellow or black, or if he has stripes like a fuckin' zebra. I'm the manager of this team, and I say he plays. What's more, I say he can make us all rich. And if any of you cannot use the money, I will see that you are all traded."^[84]

After a strike threat by some players, NL President Ford Frick and Baseball Commissioner Happy Chandler let it be known that any striking players would be suspended. Robinson received significant encouragement from several major league players, including Dodgers teammate Pee Wee Reese who said, "You can hate a man for many reasons. Color is not one of them."^[85] That year, Robinson won the inaugural Major League Baseball Rookie of the Year Award (separate NL and AL Rookie of the Year honors were not awarded until 1949).^[86]

Less than three months later, Larry Doby became the first African-American to break the color barrier in the American League with the Cleveland Indians.^[87] The next year, a number of other black players entered the major leagues. Satchel Paige was signed by the Indians and the Dodgers added star catcher Roy Campanella and Don Newcombe, who was later the first winner of the Cy Young Award for his outstanding pitching.^[88]

MLB banned the signing of women to contracts in 1952, but that ban was lifted in 1992.^[89] As of 2019, there have been no female MLB players.

From 1903 to 1953, the two major leagues consisted of two eight-team leagues. The 16 teams were located in ten cities, all in the northeastern and midwestern United States: New York City had three teams and Boston, Chicago, Philadelphia, and St. Louis each had two teams. St. Louis was the southernmost and westernmost city with a major league team. The longest possible road trip, from Boston to St. Louis, took about 24 hours by railroad. In 1953, the NL's Boston Braves became the Milwaukee Braves. In 1954, the St. Louis Browns became the Baltimore Orioles. In 1955, the Philadelphia Athletics became the Kansas City Athletics.

Baseball experts consider the Brooklyn/Los Angeles Dodgers' boss Walter O'Malley to be "perhaps the most influential owner of baseball's early expansion era."^[90] Before the 1958 Major League Baseball season, he moved the Brooklyn Dodgers to Los Angeles.^[91] When O'Malley moved the Dodgers from Brooklyn, he appeared on the cover of Time.^[92] O'Malley was also influential in persuading the rival New York Giants to move west to become the San Francisco Giants. The Giants were already suffering from slumping attendance records at their aging ballpark, the Polo Grounds. Had the Dodgers moved out west alone, the St. Louis Cardinals—1,600 mi (2,575 km) away^[93][94]—would have been the closest NL team. The joint move made West Coast road trips economical for visiting teams.^[95] O'Malley invited San Francisco Mayor George Christopher to New York to meet with Giants owner Horace Stoneham.^[95] Stoneham was considering moving the Giants to Minnesota,^[96] but he was convinced to join O'Malley on the West Coast at the end of 1957. The meetings between Stoneham, Christopher and O'Malley occurred against the wishes of Ford Frick, the Commissioner of Baseball.^[97] The dual moves were successful for both franchises—and for MLB.^[91] The Dodgers set a single-game MLB attendance record in their first home appearance with 78,672 fans.^[95]

In 1961, the first Washington Senators franchise moved to Minneapolis–St. Paul to become the Minnesota Twins. Two new teams were added to the American League at the same time: the Los Angeles Angels (who soon moved from downtown L.A. to nearby Anaheim) and a new Washington Senators franchise. The NL added the Houston Astros and the New York Mets in 1962. The Astros (known as the "Colt .45s" during their first three seasons) became the first southern major league franchise since the Louisville Colonels folded in 1899 and the first franchise to be located along the Gulf Coast. The Mets established a reputation for futility by going 40–120 during their first season of play in the nation's media capital—and by playing only a little better in subsequent campaigns—but in their eighth season (1969) the Mets became the first of the 1960s expansion teams to play in the postseason, culminating in a World Series title over the heavily favored Baltimore Orioles.

In 1966, the major leagues moved to the "Deep South" when the Braves moved to Atlanta. In 1968, the Kansas City Athletics moved west to become the Oakland Athletics. In 1969, the American and National Leagues both added two expansion franchises. The American League added the Seattle Pilots (who became the Milwaukee Brewers after one disastrous season in Seattle) and the Kansas City Royals. The NL added the first Canadian franchise, the Montreal Expos, as well as the San Diego Padres.

In 1972, the second Washington Senators moved to the Dallas–Fort Worth metroplex to become the Texas Rangers. In 1977, baseball expanded again, adding a second Canadian team, the Toronto Blue Jays, as well as the Seattle Mariners. Subsequently, no new teams were added until the 1990s and no teams moved until 2005.

By the late 1960s, the balance between pitching and hitting had swung in favor of the pitchers. In 1968—later nicknamed "the year of the pitcher"^[98]—Boston Red Sox player Carl Yastrzemski won the American League batting title with an average of just .301, the lowest in the history of Major League Baseball.^[99] Detroit Tigers pitcher Denny McLain won 31 games, making him the only pitcher to win 30 games in a season since Dizzy Dean in 1934.^[100] St. Louis Cardinals starting pitcher Bob Gibson achieved an equally remarkable feat by allowing an ERA of just 1.12.^[101]

Following these pitching performances, in December 1968 the MLB Playing Rules Committee voted to reduce the strike zone from knees to shoulders to top of knees to armpits and lower the pitcher's mound from 15 to 10 inches, beginning in the 1969 season.^[102]

In 1973, the American League, which had been suffering from much lower attendance than the National League, sought to increase scoring even further by initiating the designated hitter (DH) rule.^[103]

Throughout the 1960s and 1970s, as baseball expanded, NFL football had been surging in popularity, making it economical for many of these cities to build multi-purpose stadiums instead of single-purpose baseball fields. Because of climate and economic issues, many of these facilities had playing surfaces made from artificial turf, as well as the oval designs characteristic of stadiums designed to house both baseball and football.^[104] This often resulted in baseball fields with relatively more foul territory than older stadiums. These characteristics changed the nature of professional baseball, putting a higher premium on speed and defense over home-run hitting power, since the fields were often too big for teams to expect to hit many home runs and foul balls hit in the air could more easily be caught for outs.

Teams began to be built around pitching—particularly their bullpens—and speed on the basepaths. Artificial surfaces meant balls traveled quicker and bounced higher, so it became easier to hit ground balls "in the hole" between the corner and middle infielders. Starting pitchers were no longer expected to throw complete games; it was enough for a starter to pitch 6–7 innings and turn the game over to the team's closer, a position which grew in importance over these decades. As stolen bases increased, home run totals dropped. After Willie Mays hit 52 home runs in 1965, only one player (George Foster) reached that mark until the 1990s.

During the 1980s, baseball experienced a number of significant changes the game had not seen in years. Home runs were on the decline throughout the decade, with players hitting only 40 home runs just 13 times and no one hitting more than 50 home runs in a season for the first time since the Dead-ball era (1900—1919).^[105][106] Teams instead focused on building their rosters around speed and defense.

The 1981 Major League Baseball strike from June 12 until July 31 forced the cancellation of 713 total games and resulted in a split-season format.

In 1985, Pete Rose broke Ty Cobb's all-time hits record with his 4,192nd hit, and in 1989 Rose received a lifetime ban from baseball as a result of betting on baseball games while manager of the Cincinnati Reds. Rose was the first person to receive a lifetime ban from baseball since 1943.^[105][107] 1985 also saw the Pittsburgh drug trials which involved players who were called to testify before a grand jury in Pittsburgh related to cocaine trafficking. Eleven players were officially suspended, but all the suspensions were commuted in exchange for fines, drug testing, and community service.

Routinely in the late 1990s and early 2000s, baseball players hit 40 or 50 home runs in a season, a feat that was considered rare even in the 1980s. It has since become apparent that at least some of this power surge was a result of players using steroids and other performance-enhancing drugs.

In 1993, the NL added the Florida Marlins in the Miami area and the Colorado Rockies in Denver. In 1998, the Brewers switched leagues by joining the National League and two new teams were added: the NL's Arizona Diamondbacks in Phoenix and the American League's Tampa Bay Devil Rays in St. Petersburg, Florida.

After the 2001 season, the team owners voted in favor of contraction. Several MLB teams had been considered for elimination in early talks about contraction, but the Montreal Expos and the Minnesota Twins were the two teams that came closest to folding under the plan. Plans for MLB contraction were halted when the Twins landlord was awarded a court injunction that required the team to play its 2002 home games at their stadium. MLB owners agreed to hold off on reducing the league's size until at least 2006.^[108]

The Montreal Expos became the first franchise in over three decades to move when they became the Washington Nationals in 2005. This move left Canada with just one team, but it also returned baseball to the United States capital city after a 33-year absence. This franchise shift, like many previous ones, involved baseball's return to a city which had been previously abandoned. Not counting the short-lived Federal League, Montreal is the only city granted an MLB franchise since 1901 that does not currently host a team.

A baseball uniform is a type of uniform worn by baseball players, and by some non-playing personnel, such as field managers and coaches. It is worn to indicate the person's role in the game and—through the use of logos, colors, and numbers—to identify the teams and their players, managers, and coaches.^[109]

Traditionally, home uniforms display the team name on the front, while away uniforms display the team's home location. In modern times, however, exceptions to this pattern have become common, with teams using their team name on both uniforms.^[110] Most teams also have one or more alternate uniforms, usually consisting of the primary or secondary team color on the vest instead of the usual white or gray.^[110] In the past few decades throwback uniforms have become popular.^[111]

The New York Knickerbockers were the first baseball team to use uniforms, taking the field on April 4, 1849, in pants made of blue wool, white flannel shirts (jerseys) and straw hats.^{[112][113][114]} Caps and other types of headgear have been a part of baseball uniforms from the beginning.^[115][116] Baseball teams often wore full-brimmed straw hats or no cap at all since there was no official rule regarding headgear.^[117] Under the 1882 uniform rules, players on the same team wore uniforms of different colors and patterns that indicated which position they played.^[118]

In the late 1880s, the Detroit Wolverines and Washington Nationals of the National League and the Brooklyn Bridegrooms of the American Association were the first to wear striped uniforms.^[119] By the end of the 19th century, teams began the practice of having two different uniforms, one for when they played at home in their own baseball stadium and a different one for when they played away (on the road) at the other team's ballpark.^[110] It became common to wear white pants with a white color vest at home and gray pants with a gray or solid (dark) colored vest when away.^[110] By 1900, both home and away uniforms were standard across the major leagues.^[120]

Spring training is a series of practices and exhibition games preceding the start of the regular season. Teams hold training camps in the states of Arizona and Florida, where the early warm weather allows teams to practice and play without worrying about late winter cold. Spring training allows new players to audition for roster and position spots, and gives existing team players practice time prior to competitive play. The teams that hold spring training in Arizona are grouped into the Cactus League,^[121] while teams that hold camp in Florida are known as the Grapefruit League.^[122] Spring training has always attracted fan attention, drawing crowds who travel to the warmer climates to enjoy the weather and watch their favorite teams play, and spring training usually coincides with spring break for many college students. Autograph seekers also find greater access to players during spring training.

Spring training typically lasts almost two months, starting in mid February and running until just before the season opening day, traditionally the first week of April. As pitchers benefit from a longer training period, pitchers and catchers begin spring training several days before the rest of the team.^[123]

The current MLB regular season, consisting of 162 games per team, typically begins on the first Sunday in April and ends on the first Sunday in October. Each team's schedule is typically organized into three-game series, with occasional two- or four-game series.^[124] Postponed games or continuations of suspended games can result in an ad hoc one-game or five-game series. A team's series are organized into homestands and road trips that group multiple series together. Teams generally play games five to seven days per week, commonly having Monday or Thursday as an off day. Frequently, games are scheduled at night. Sunday games are generally played during the afternoon, allowing teams to travel to their next destination prior to a Monday night game. In addition, teams will play day games frequently on Opening Day, holidays, and getaway days.

Each team plays 19 games against each of its four divisional opponents. It plays one home series and one away series, amounting to six or seven games, against the 10 other teams in its league. A team also plays one of the divisions in the other league, rotating each year, with two opponents in a three-game home series, two in a three-game away series, and one with four games split between home and away. Furthermore, each team has an interleague "natural rival" (in many cases its counterpart in the same metro area) with which it plays two home games and two away games each year.

With an odd number of teams in each league (15), it is necessary to have two teams participate in interleague play for most days in the season, except when two or more teams have a day off. Each team plays 20 interleague games throughout the season, usually with just one interleague game per day, but for one weekend in late May all teams will participate in an interleague series. Use of the DH rule is determined by the home team's league rules. Before 2013, interleague play was structured differently: there would be one weekend in mid-May and another period consisting typically of the last two-thirds of June in which all teams played interleague games (save for two NL teams each day), and no interleague games were scheduled outside those dates. (Before 2013, season-long interleague play was not necessary, because each league had an even number of teams. In 2013, the Houston Astros moved to the American League, so that each league would have 15 teams.)

Over the course of a season, teams compete for the five playoff berths in their respective leagues. In order to secure a berth, a team must either win its division, or capture a wild card spot by having one of the two best records among the non-winners in its entire league.^[125]

After the conclusion of the 162-game season, an additional tie-breaking game (or games) may be needed to determine postseason participation.^[126]

In early-to-mid July, just after the midway point of the season, the Major League Baseball All-Star Game is held during a four-day break from the regular-season schedule. The All-Star Game features a team of players from the American League (AL)—led by the manager of the previous AL World Series team—and a team of players from the National League (NL), similarly managed, in an exhibition game. From 1959 to 1962, two games were held each season, one was held in July and one was held in August. The designated-hitter rule was used in the All-Star Game for the first time in 1989. Following games used a DH when the game was played in an AL ballpark. Since 2010, the DH rule has been in effect regardless of venue.^[127]

The first official All-Star Game was held as part of the 1933 World's Fair in Chicago, Illinois, and was the idea of Arch Ward, then sports editor for The Chicago Tribune.^[128] Initially intended to be a one-time event, its great success resulted in making the game an annual one. Ward's contribution was recognized by Major League Baseball in 1962 with the creation of the "Arch Ward Trophy", given to the All-Star Game's Most Valuable Player each year.^[129] (In 1970, it was renamed the Commissioner's Trophy, until 1985, when the name change was reversed. In 2002, it was renamed the Ted Williams Most Valuable Player Award.)

Beginning in 1947, the eight position players in each team's starting lineup have been voted into the game by fans.^[128] The fan voting was discontinued after a 1957 ballot-box-stuffing scandal in Cincinnati: seven of the eight slots originally went to Reds players, two of whom were subsequently removed from the lineup to make room for Willie Mays and Hank Aaron. Fan voting was reinstated in 1970 and has continued ever since, including Internet voting in recent years.

The 2002 contest in Milwaukee controversially ended in an 11-inning tie when both managers ran out of pitchers. In response, starting in 2003 the league which wins the All-Star game received home-field advantage in the World Series: the league champion hosted the first two games at its own ballpark as well as the last two (if necessary). The National League did not win an All-Star game and thus gain home-field advantage until 2010; it was able to overcome this disadvantage and win in three of the seven World Series from 2003 to 2009.^[130][131] This was discontinued after the 2016 season.

MLB All-Stars from both leagues have worn uniforms from their respective teams at the game with one exception. In the 1933 All-Star Game, the National League All-Star Team members wore special gray uniforms with "National League" written in navy blue letters across the front of the jersey.^[132][133]

When the regular season ends after the first Sunday in October (or the last Sunday in September), ten teams enter the postseason playoffs. These ten teams consist of six teams that are division champions by earning the best regular season overall win-loss record for their respective divisions, and four who are "wild-card" teams that are each one of two teams in their respective leagues who have earned the best regular season win-loss record, but are not division champions. Four rounds of series of games are played to determine the champion:

Within each league, the division winners are the #1, #2 and #3 seeds, based on win–loss records. The team with the best record among non division winners will be the first wildcard and the #4 seed. The team with the second best record among non division winners will be the second wildcard and the #5 seed. In the wildcard round, the #5 seed will play at the #4 seed in a one-game playoff. For the division series, the matchup will be the #1 seed against the Wild Card Game winner and the #2 seed against the #3 seed.^[134] Since 2017, home-field advantage in the World Series is determined by regular-season records of the two league champions, replacing a system used for the prior 14 seasons where the champion of the league that won the All-Star Game would receive home-field advantage.

Because each postseason series is split between the home fields of the two teams, home-field advantage does not usually play a large role in the postseason unless the series goes to its maximum number of games, giving one team an additional game at home. However, the first two games of a postseason series are hosted by the same team. That team may have an increased chance of starting the series with two wins,^[135] thereby gaining some momentum for the rest of the series.^[136]

The DH rule in the World Series is only used in games played at the American League Champions' home field. National League home games will play the traditional rules with each teams pitching staff batting.^[137]

Since 1986 an All-Star team from MLB is sent to a biennial end-of-the-season tour of Japan, dubbed as MLB Japan All-Star Series, playing exhibition games in a best-of format against the All-Stars from Nippon Professional Baseball (NPB) or recently as of 2014 their national team Samurai Japan.

In 2008, MLB played the MLB China Series in the People's Republic of China. It was a series of two spring-training games between the San Diego Padres and Los Angeles Dodgers. The games were an effort to popularize baseball in China.^[138]

MLB played the MLB Taiwan All-Star Series in Taiwan in November 2011. It was a series of five exhibition games played by a team made up of MLB players called the MLB All-Stars and the Chinese Taipei national baseball team. The MLB All-Stars swept the series, five games to zero.^[139] At the end of the 2011 season, it was announced that the Seattle Mariners and the Oakland Athletics would play their season openers in Japan.^[140] In October 2013, Phil Rogers of the Chicago Tribune wrote that MLB was considering postseason all-star tours in Taiwan and Korea; baseball is increasing in popularity in both countries.^[141]

The Arizona Diamondbacks opened the 2014 season against Los Angeles Dodgers on March 22–23 in Australia.^[142] The teams played each other at the historic Sydney Cricket Ground, which has a seating capacity of 46,000. The two games represented the first MLB regular-season play held in that country. The games counted as home games for the Diamondbacks, so they played 79 home games at Chase Field.^[143]

In 2019, the Red Sox were the home team in the inaugural MLB London Series, a regular season two-game series against the Yankees. The games were played on June 29–30 at London Stadium with the Yankees winning both games. The pair were the first regular season MLB games held in Europe.

Together with the World Baseball Softball Confederation, MLB sponsors the World Baseball Classic, an international baseball tournament contested by national teams.

In 1998, both Mark McGwire and Sammy Sosa hit more than the long-standing single-season MLB record of 61 home runs. Barry Bonds topped the record in 2001 with 73 home runs. McGwire, Bonds and Sosa became the subjects of speculation regarding the use of performance-enhancing substances. McGwire later admitted that he used a steroid hormone that was still legal in baseball during the 1998 season.^[145] Baseball's original steroid testing policy, in effect from 2002 to 2005, provided for penalties ranging from a ten-game suspension for a first positive test to a one-year suspension for a fourth positive test. Players were tested at least once per year, with the chance that several players could be tested many times per year.^[146]

A 2006 book, Game of Shadows by San Francisco Chronicle investigative reporters Lance Williams and Mark Fainaru-Wada, chronicled alleged extensive use of performance enhancers, including several types of steroids and growth hormone by baseball superstars Barry Bonds, Gary Sheffield, and Jason Giambi. Former Senate Majority Leader George Mitchell was appointed by Selig on March 30, 2006^[147] to investigate the use of performance-enhancing drugs in MLB. The appointment was made after several influential members of the U.S. Congress made negative comments about both the effectiveness and honesty of MLB's drug policies and Commissioner Selig.^[148]

The day before the Mitchell Report was to be released in 2007, Selig said, "I haven't seen the report yet, but I'm proud I did it."^[149][150] The report said that after mandatory random testing began in 2004, HGH treatment for athletic enhancement became popular among players, as HGH is not detectable in tests. It pointed out that HGH is likely a placebo with no performance-enhancing effects.^[151] The report included substance use allegations against at least one player from each MLB team.^[152]

According to ESPN, some people questioned whether Mitchell's director role with the Boston Red Sox created a conflict of interest, especially because no "prime [Sox] players were in the report."^[153] The report named several prominent Yankees who were parts of World Series clubs; there is a long-running and fierce Yankees–Red Sox rivalry. Former U.S. prosecutor John M. Dowd brought up Mitchell's conflict of interest,^[154] but he later said that the former senator had done a good job.^[155] Mitchell acknowledged that his "tight relationship with Major League Baseball left him open to criticism",^[156] but he said that readers who examine the report closely "will not find any evidence of bias, of special treatment of the Red Sox".^[156]

On January 10, 2013, MLB and the players union reached an agreement to add random, in-season HGH testing. They also agreed to implement a new test to reveal the use of testosterone for the 2013 season.^[157] The current MLB drug policy provides for an 80-game suspension for a first positive test, a 162-game suspension for a second positive test, and a lifetime suspension for a third positive test.^[158] In 2009, allegations surfaced against Alex Rodriguez and David Ortiz, and Manny Ramirez received a 50-game suspension after testing positive for banned substances. In early April 2011, Ramirez retired from baseball rather than face a 100-game suspension for his second positive steroid test.^[159] He would later unretire, having the suspension dropped to 50 games, and would serve those in 2012.

Several networks televise baseball games, including Fox, ESPN, and MLB Network. Since 2008, Fox Sports has broadcast MLB games on Fox Saturday Baseball throughout the entire season; Fox previously only broadcast games from May to September.^[160] Fox also holds rights to the All-Star Game each season. Fox also alternates League Championship Series broadcasts, broadcasting the American League Championship Series (ALCS) in odd-numbered years and the National League Championship Series (NLCS) in even-numbered years. Fox broadcasts all games of the World Series. ESPN continues to broadcast MLB games through 2013 as well, beginning with national Opening Day coverage.^[161] ESPN broadcasts Sunday Night Baseball, Monday Night Baseball, Wednesday Night Baseball, and Baseball Tonight. ESPN also has rights to the Home Run Derby at the All-Star Game each July.^[150]

TBS airs Sunday afternoon regular season games (non-exclusive) nationally. In 2007, TBS began its exclusive rights to any tiebreaker games that determine division or wild card champions; However, in 2018, the two tiebreaker games were broadcast on ESPN. it also airs exclusive coverage of the Division Series round of the playoffs.^[162] TBS carries the League Championship Series that are not included under Fox's television agreement; TBS shows the NLCS in odd-numbered years and the ALCS in even-numbered years.^[163]

In January 2009, MLB launched the MLB Network, featuring news and coverage from around the league, and airing 26 live games in the 2009 season.^[164] Each team also has local broadcasts for all games not carried by Fox on Saturdays or ESPN on Sunday nights. These games are typically split between a local broadcast television station and a local or regional sports network (RSN), though some teams only air local games through RSNs or through their own team networks. As Canada only contains one team, Sportsnet broadcasts Toronto Blue Jays games nationally.^[165] The channel is owned by Rogers Communications, who is also the parent company of the Blue Jays.^[165] Sportsnet also televises Fox's Saturday afternoon games, the All-Star Game, playoff games, and the World Series.^[165][166] In April 2011, TSN2 began carrying ESPN Sunday Night Baseball in Canada.^[167]

MLB has several blackout rules.^[168] A local broadcaster has priority to televise games of the team in their market over national broadcasters. For example, at one time TBS showed many Atlanta Braves games nationally and internationally in Canada. Fox Sports Networks also show many games in other areas. If the Braves played a team that FSN or another local broadcaster showed, the local station will have the broadcast rights for its own local market, while TBS would have been blacked out in the same market during the game. A market that has a local team playing in a weekday ESPN or ESPN2 game and is shown on a local station will see ESPNews, or, in the past, another game scheduled on ESPN or ESPN2 at the same time (if ESPN or ESPN2 operates a regional coverage broadcasting and operates a game choice), or will be subject to an alternative programming feed.^[169] MLB's streaming Internet video service is also subject to the same blackout rules.

ESPN Radio holds national broadcast rights and broadcasts Sunday Night Baseball weekly throughout the season in addition to all playoff games.^[170] The rights to the World Series are exclusive to ESPN.

In addition, each team employs its own announcers, who broadcast during the regular season. Most teams operate regional networks to cover their fan bases; some of these supposedly regional networks (such as the New York Yankees Radio Network) have a national reach with affiliates located across the United States.^[171] Major League Baseball has an exclusive rights deal with XM Satellite Radio, which includes the channel MLB Network Radio and live play-by-play of all games.^[172] Many teams also maintain a network of stations that broadcast their games in Spanish; the former Montreal Expos broadcast their games in both English and French.

MLB games are also broadcast live on the internet. All television and radio broadcasts of games are available via subscription to MLB.tv at Major League Baseball's website, MLB.com, and radio-only broadcasts are also available via subscription to MLB.com Gameday Audio.^[173] Radio station affiliates are officially forbidden from streaming games through their Internet feeds. Blackout rules are still applied for live television broadcasts, but not radio broadcasts.

ESPN Deportes televises a large number of MLB games in Spanish throughout Latin America.^[174] Wapa 2 airs games in Puerto Rico, including spring training games and most of the World Baseball Classic games involving the team from Puerto Rico.^[175] In Brazil, ESPN Brasil has exclusive rights on TV (ESPN and ESPN+) and Internet (WatchESPN),^[176] with Fox Sports also broadcasting some games.

Five in the United Kingdom previously screened MLB games, including the All-Star Game and the postseason games, on Sunday and Wednesday usually starting at 1 a.m. BST. Most recently, Johnny Gould and Josh Chetwynd presented MLB on Five on that station.^[177] The channel covered baseball beginning on its opening night in 1997, but for financial reasons, the decision was made not to pick up MLB for the 2009 season.^[178] ESPN UK show live and recorded games several times a week—it is available with BT Sport and (on a subscriber-basis) Virgin Media in the UK.^[179] ESPN America televised a large number of games in the UK and dozens of other countries; in May 2013, ESPN announced that it would shut down the channel on July 31, 2013.^[180][181]

In Australia, MLB games are regularly shown on ESPN Australia (subscription).^[182]

In the Middle East & North Africa, MLB games are broadcast on beIN Sports channels.

In Italy MLB games are broadcast on DAZN