The United States district courts are the general trial courts of the United States federal court system. Both civil and criminal cases are filed in the district court, which is a court of law, equity, and admiralty. There is a United States bankruptcy court associated with each United States district court. Each federal judicial district has at least one courthouse, and many districts have more than one. The formal name of a district court is "the United States District Court for" the name of the district—for example, the United States District Court for the Eastern District of Missouri.

In contrast to the Supreme Court, which was established by Article III of the Constitution, the district courts were established by Congress.^[1] There is no constitutional requirement that district courts exist at all. Indeed, after the ratification of the Constitution, some opponents of a strong federal judiciary urged that, outside jurisdictions under direct federal control, like Washington, D.C., and the territories, the federal court system be limited to the Supreme Court, which would hear appeals from state courts. This view did not prevail, however, and the first Congress created the district court system that is still in place today.

There is at least one judicial district for each state, the District of Columbia, and Puerto Rico. The territories (insular areas) of Guam, the Northern Mariana Islands, and the United States Virgin Islands each have one territorial court; these courts are called "district courts" and exercise the same jurisdiction as district courts,^[4][5] but differ from district courts in that territorial courts are Article IV courts, with judges who serve ten-year terms rather than the lifetime tenure of judges of Article III courts, such as the district court judges.^[5] American Samoa does not have a district court or a federal territorial court, and so federal matters there are sent to either the District of Columbia or Hawaii.^[6]

There are 89 districts in the 50 states, with a total of 94 districts including territories.^[7]

There are other federal trial courts that have nationwide jurisdiction over certain types of cases, but the district court also has concurrent jurisdiction over many of those cases, and the district court is the only one with jurisdiction over civilian criminal cases.

The United States Court of International Trade addresses cases involving international trade and customs issues. The United States Court of Federal Claims has exclusive jurisdiction over most claims for money damages against the United States, including disputes over federal contracts, unlawful takings of private property by the federal government, and suits for injury on federal property or by a federal employee. The United States Tax Court has jurisdiction over contested pre-assessment determinations of taxes.

A judge of a United States district court is officially titled a "United States District Judge". Other federal judges, including circuit judges and Supreme Court Justices, can also sit in a district court upon assignment by the chief judge of the circuit or by the Chief Justice of the United States. The number of judges in each district court (and the structure of the judicial system generally) is set by Congress in the United States Code. The President appoints the federal judges for terms of good behavior (subject to the advice and consent of the Senate), so the nominees often share at least some of his or her convictions. In states represented by a senator of the President's party, the senator (or the more senior of them if both senators are of the president's party) has substantial input into the nominating process, and through a tradition known as senatorial courtesy can exercise an unofficial veto over a nominee unacceptable to the senator.

With the exception of the territorial courts (Guam, the Northern Mariana Islands, and the Virgin Islands), federal district judges are Article III judges appointed for life, and can be removed involuntarily only when they violate the standard of "good behavior". The sole method of involuntary removal of a judge is through impeachment by the United States House of Representatives followed by a trial in the United States Senate and a conviction by a two-thirds vote. Otherwise, a judge, even if convicted of a felony criminal offense by a jury, is entitled to hold office until retirement or death. In the history of the United States, only twelve judges have been impeached by the House, and only seven have been removed following conviction in the Senate. (For a table that includes the twelve impeached judges, see Impeachment in the United States.)

A judge who has reached the age of 65 (or has become disabled) may retire or elect to go on senior status and keep working. Such senior judges are not counted in the quota of active judges for the district and do only whatever work they are assigned by the chief judge of the district, but they keep their offices (called "chambers") and staff, and many of them work full-time. A federal judge is addressed in writing as "The Honorable John/Jane Doe" or "Hon. John/Jane Doe" and in speech as "Judge" or "Judge Doe" or, when presiding in court, "Your Honor".

District judges usually concentrate on managing their court's overall caseload, supervising trials, and writing opinions in response to important motions like the motion for summary judgment. Since the 1960s, routine tasks like resolving discovery disputes can, in the district judge's discretion, be referred to magistrate judges. Magistrate judges can also be requested to prepare reports and recommendations on contested matters for the district judge's consideration or, with the consent of all parties, to assume complete jurisdiction over a case including conducting the trial.

Federal magistrate judges are appointed by each district court pursuant to statute. They are appointed for an eight-year term and may be reappointed for additional eight-year terms. A magistrate judge may be removed "for incompetency, misconduct, neglect of duty, or physical or mental disability".^[8] A magistrate judgeship may be a stepping stone to a district judgeship nomination.

As of 2010, there were 678 authorized district court judgeships.^[9]

Each district court appoints a clerk, who is responsible for overseeing filings made with the court, maintaining the court's records, processing fees, fines, and restitution, and managing the non-judicial work of the court, including information technology, budget, procurement, human resources, and financial. Clerks may appoint deputies, clerical assistants, and employees to carry out the work of the court. The clerk of each district court must reside in the district for which the clerk is appointed, except that the clerk of the District of Columbia and the clerk of the Southern District of New York may reside within twenty miles of their respective districts.

The Judiciary Act of 1789 authorized the Supreme Court and the judge of each U.S. District Court to appoint a clerk to assist with the administration of federal judicial business in those courts. The clerk for each district court was to also serve as clerk of the corresponding circuit court. The Judiciary Act required each clerk to issue the writs summoning jurors and "to record the decrees, judgments and determinations of the court of which he is clerk."

The Judicial Code (28 U.S.C. § 751) provides that the clerk is appointed, and may be removed, by the court. The clerk's duties are prescribed by the statute, by the court's customs and practices, and by policy established by the Judicial Conference of the United States. The clerk is appointed by order of the court en banc to serve the entire court. The role of the clerk and deputies or assistants should not be confused with the court's law clerks, who assist the judges by conducting research and preparing drafts of opinions.

To be eligible to serve as a clerk, a person must have a minimum of 10 years of progressively responsible administrative experience in public service or business that provides a thorough understanding of organizational, procedural, and human aspects of managing an organization, and at least 3 of the 10 years must have been in a position of substantial management responsibility. An attorney may substitute the active practice of law on a year-for-year basis for the management or administrative experience requirement. Clerks do not have to be licensed attorneys, but some courts specify that a law degree is a preference for employment.

Unlike some state courts, the power of federal courts to hear cases and controversies is strictly limited. Federal courts may not decide every case that happens to come before them. In order for a district court to entertain a lawsuit, Congress must first grant the court subject matter jurisdiction over the type of dispute in question.

The district courts exercise original jurisdiction over—that is, they are empowered to conduct trials in—the following types of cases:

For most of these cases, the jurisdiction of the federal district courts is concurrent with that of the state courts. In other words, a plaintiff can choose to bring these cases in either a federal district court or a state court. Congress has established a procedure whereby a party, typically the defendant, can "remove" a case from state court to federal court, provided that the federal court also has original jurisdiction over the matter (meaning that the case could have been filed in federal court initially).^[16] If the party that initially filed the case in state court believes that removal was improper, that party can ask the district court to "remand" the case to the state court system. For certain matters, such as patent and copyright infringement disputes and prosecutions for federal crimes, the jurisdiction of the district courts is exclusive of that of the state courts, meaning that only federal courts can hear those cases.^[2]

In addition to their original jurisdiction, the district courts have appellate jurisdiction over a very limited class of judgments, orders, and decrees.^[17]

In order to represent a party in a case in a district court, a person must be an attorney at law and generally must be admitted to the bar of that particular court. The United States usually does not have a separate bar examination for federal practice (except with respect to patent practice before the United States Patent and Trademark Office). Admission to the bar of a district court is generally available to any attorney who is admitted to practice law in the state where the district court sits.^[3]

56 districts (around 60% of all district courts) require an attorney to be admitted to practice in the state where the district court sits. The other 39 districts (around 40% of all district courts) extend admission to certain lawyers admitted in other states, although conditions vary from court to court. For example, the district courts in New York City (Southern District of New York and Eastern District of New York) extend admission to attorneys admitted to the bar in Connecticut or Vermont and to the district court in that state, but otherwise require attorneys to be admitted to the New York bar. Only 13 districts extend admission to attorneys admitted to any U.S. state bar.^[18][19]

The attorney generally submits an application with a fee and takes the oath of admission. Local practice varies as to whether the oath is given in writing or in open court before a judge of the district. A "sponsor" admitted to the court's bar is often required. Several district courts require attorneys seeking admission to their bars to take an additional bar examination on federal law, including the following: the Southern District of Ohio,^[20] the Northern District of Florida,^[21] and the District of Puerto Rico.^[22]

Pro hac vice admission is also available in most federal district courts on a case-by-case basis. Most district courts require pro hac vice attorneys to associate with an attorney admitted to practice before the court.^[18]

Generally, a final ruling by a district court in either a civil or a criminal case can be appealed to the United States court of appeals in the federal judicial circuit in which the district court is located, except that some district court rulings involving patents and certain other specialized matters must be appealed instead to the United States Court of Appeals for the Federal Circuit, and in a very few cases the appeal may be taken directly to the United States Supreme Court.

The Central District of California is the largest federal district by population;^[23] it includes all five counties that make up Greater Los Angeles. By contrast, New York City and the surrounding metropolitan area are divided between the Southern District of New York (which includes Manhattan, The Bronx and Westchester County) and the Eastern District of New York (which includes Brooklyn, Queens, Staten Island, Nassau County and Suffolk County). New York suburbs in Connecticut and New Jersey are covered by the District of Connecticut and District of New Jersey, respectively.

The Southern District of New York and the Central District of California are the largest federal districts by number of judges, with 28 and 27, respectively.^[24]

In 2007, the busiest district courts in terms of criminal federal felony filings were the District of New Mexico, Western District of Texas, Southern District of Texas, and the District of Arizona. These four districts all share the border with Mexico.^[25] A crackdown on illegal immigration resulted in 75 percent of the criminal cases filed in the 94 district courts in 2007 being filed in these four districts and the other district that borders Mexico, the Southern District of California.^[26] The busiest patent litigation court is the United States District Court for the Eastern District of Texas, with the most patent lawsuits filed there nearly every year.^[27]

Most extinct district courts have disappeared by being divided into smaller districts. The following courts were subdivided out of existence: Alabama, Arkansas, California, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky, Louisiana, Michigan, Mississippi, Missouri, New York, North Carolina, Ohio, Pennsylvania, Tennessee, Texas, Virginia, Washington, West Virginia, Wisconsin.

On rare occasions, an extinct district court was extinguished by merging it with other district courts. In every case except one, this has restored a district court that had been subdivided:

There are a few additional extinct district courts that fall into neither of the above two patterns.