A parliamentary system or parliamentary democracy is a system of democratic governance of a state (or subordinate entity) where the executive derives its democratic legitimacy from its ability to command the confidence of the legislature, typically a parliament, and is also held accountable to that parliament. In a parliamentary system, the head of state is usually a person distinct from the head of government. This is in contrast to a presidential system, where the head of state often is also the head of government and, most importantly, the executive does not derive its democratic legitimacy from the legislature.

Countries with parliamentary democracies may be constitutional monarchies, where a monarch is the head of state while the head of government is almost always a member of parliament (such as the United Kingdom, Denmark, Sweden, and Japan), or parliamentary republics, where a mostly ceremonial president is the head of state while the head of government is regularly from the legislature (such as Ireland, Germany, India, and Italy). In a few parliamentary republics, such as Botswana, South Africa, and Suriname, among some others, the head of government is also head of state, but is elected by and is answerable to parliament. In bicameral parliaments, the head of government is generally, though not always, a member of the lower house.

Parliamentarianism is the dominant form of government in Europe, with 32 of its 50 sovereign states being parliamentarian. It is also common in the Caribbean, being the form of government of 10 of its 13 island states, and in Oceania. Elsewhere in the world, parliamentary countries are less common, but they are distributed through all continents, most often in former colonies of the British Empire that subscribe to a particular brand of parliamentarianism known as the Westminster system.

Since ancient times, when societies were tribal, there were councils or a headman whose decisions were assessed by village elders.

Eventually, these councils have slowly evolved into the modern parliamentary system.

The first parliaments date back to Europe in the Middle Ages: specifically in 1188 Alfonso IX, King of Leon (Spain) convened the three states in the Cortes of León.^[1][2] An early example of parliamentary government developed in today's Netherlands and Belgium during the Dutch revolt (1581), when the sovereign, legislative and executive powers were taken over by the States General of the Netherlands from the monarch, King Philip II of Spain. The modern concept of parliamentary government emerged in the Kingdom of Great Britain between 1707–1800 and its contemporary, the Parliamentary System in Sweden between 1721–1772.

In England, Simon de Montfort is remembered as one of the fathers of representative government for convening two famous parliaments.^[3][4] The first, in 1258, stripped the king of unlimited authority and the second, in 1265, included ordinary citizens from the towns.^[5] Later, in the 17th century, the Parliament of England pioneered some of the ideas and systems of liberal democracy culminating in the Glorious Revolution and passage of the Bill of Rights 1689.^[6][7]

In the Kingdom of Great Britain, the monarch, in theory, chaired cabinet and chose ministers. In practice, King George I's inability to speak English led the responsibility for chairing cabinet to go to the leading minister, literally the prime or first minister, Robert Walpole. The gradual democratisation of parliament with the broadening of the voting franchise increased parliament's role in controlling government, and in deciding whom the king could ask to form a government. By the 19th century, the Great Reform Act of 1832 led to parliamentary dominance, with its choice invariably deciding who was prime minister and the complexion of the government.^[8][9]

Other countries gradually adopted what came to be called the Westminster Model of government, with an executive answerable to parliament, and exercising, in the name of the head of state, powers nominally vested in the head of state. Hence the use of phrases like Her Majesty's government or His Excellency's government. Such a system became particularly prevalent in older British dominions, many of which had their constitutions enacted by the British parliament; such as Australia, New Zealand, Canada, the Irish Free State and the Union of South Africa. Some of these parliaments were reformed from, or were initially developed as distinct from their original British model: the Australian Senate, for instance, has since its inception more closely reflected the US Senate than the British House of Lords; whereas since 1950 there is no upper house in New Zealand.

Democracy and parliamentarianism became increasingly prevalent in Europe in the years after World War I, partially imposed by the democratic victors, the United States, Great Britain and France, on the defeated countries and their successors, notably Germany's Weimar Republic and the new Austrian Republic. Nineteenth-century urbanisation, the Industrial Revolution and modernism had already fuelled the political left's struggle for democracy and parliamentarianism for a long time. In the radicalised times at the end of World War I, democratic reforms were often seen as a means to counter popular revolutionary currents.

A parliamentary system may be either bicameral, with two chambers of parliament (or houses) or unicameral, with just one parliamentary chamber. A bicameral parliament usually consists of a directly elected lower house with the power to determine the executive government, and an upper house which may be appointed or elected through a different mechanism from the lower house.

Scholars of democracy such as Arend Lijphart distinguish two types of parliamentary democracies: the Westminster and Consensus systems.^[10]

Implementations of the parliamentary system can also differ as to how the prime minister and government are appointed and whether the government needs the explicit approval of the parliament, rather than just the absence of its disapproval.

Some countries such as India also require the prime minister to be a member of the legislature, though in other countries this only exists as a convention.

Furthermore, there are variations as to what conditions exist (if any) for the government to have the right to dissolve the parliament:

The parliamentary system can be contrasted with a presidential system which operates under a stricter separation of powers, whereby the executive does not form part of—nor is appointed by—the parliamentary or legislative body. In such a system, parliaments or congresses do not select or dismiss heads of governments, and governments cannot request an early dissolution as may be the case for parliaments. There also exists the semi-presidential system that draws on both presidential systems and parliamentary systems by combining a powerful president with an executive responsible to parliament: for example, the French Fifth Republic.

Parliamentarianism may also apply to regional and local governments. An example is the city of Oslo, which has an executive council (Byråd) as a part of the parliamentary system.

A few parliamentary democratic nations such as India,^[17] Pakistan, and Bangladesh, have enacted an anti-defection law, which prohibits a member of the legislature from switching to another party after being elected. With this law, elected representatives lose their seats in parliament if they vote contrary to the directions of their party.

One of the advantages commonly attributed to parliamentary systems is that it is faster and easier to pass legislation,^[18] as the executive branch is formed by the direct or indirect support of the legislative branch and often includes members of the legislature.

Thus the executive (as the majority party or coalition of parties in the legislature) has a majority of the votes and can pass legislation at will.

In a presidential system, the executive is often chosen independently from the legislature.

If the executive and the majority of the legislature are from different political parties, then stalemate can occur (see Divided government). Thus the executive might not be able to implement its legislative proposals. An executive in any system (be it parliamentary, presidential or semi-presidential) is chiefly voted into office on the basis of his or her party's platform/manifesto, and the same is also true of the legislature.

Some constituencies may have a popular local candidate under an unpopular leader (or the reverse), forcing a difficult choice on the electorate.

Mixed-member proportional representation, where voters cast two votes, can make this choice easier by allowing voters to cast one vote for the local candidate (at the constituency level) but also cast a second vote for another party (at the wider parliamentary level). However, many other electoral systems such as first-past-the-post voting don't allow for this and only let voters pick either a single candidate or party, without regard for the voter disliking the candidate's party or the party's local candidate.

It has been observed that the rankings of top-performing countries according to performance indices such as list of countries by GDP (nominal) per capita, Human Development Index, Global Competitiveness Report, Corruption Perceptions Index, and many more performance indexes feature most best-performing countries having parliamentary systems, while most worst-performing countries have presidential systems or strong-president semi-presidential systems. Furthermore, most of the countries that dominate top ranks of lists like the Global Liveability Ranking, the Mercer Quality of Living Survey, the Henley Passsport Index, and many such ranking lists use parliamentary systems. In contrast, the list of cities by murder rate shows an overwhelming number of cities found in countries that use presidential systems. In 2017, a paper by the University of York's economics department argued that "presidential regimes consistently produce less favourable outcomes as compared with parliamentary ones with lower output growth, higher and more volatile inflation and greater income inequality. Moreover, the magnitude of this effect is sizable."^[19]

Parliamentary government has attractive features for nations that are ethnically, racially, or ideologically divided. In a presidential system, all executive power is vested in one person, the president, whereas power is more divided in a parliamentary system with its collegial executive. In the 1989 Lebanese Taif Agreement, in order to give Muslims greater political power, Lebanon moved from a semi-presidential system with a powerful president to a system more structurally similar to classical parliamentary government. Iraq similarly disdained a presidential system out of fears that such a system would be tantamount to Shiite domination of the large Sunni minority. Afghanistan's minorities refused to go along with a presidency as strong as the Pashtuns desired.

It can also be argued that power is more evenly spread out in parliamentary government, as the government and prime minister do not have the power to make unilateral decisions, as the entire government cabinet is answerable and accountable to parliament.

Parliamentary systems are less likely to allow celebrity-based politics to fully dominate a society, unlike what often happens in presidential systems, where name-recall and popularity can catapult a celebrity, actor, or popular politician to the presidency despite such candidate's lack of competence and experience.

Some scholars like Juan Linz, Fred Riggs, Bruce Ackerman, and Robert Dahl have found that parliamentary government is less prone to authoritarian collapse. These scholars point out that since World War II, two-thirds of Third World countries establishing parliamentary governments successfully made the transition to democracy. By contrast, no Third World presidential system successfully made the transition to democracy without experiencing coups and other constitutional breakdowns. A 2001 World Bank study found that parliamentary systems are associated with less corruption,^[20] which is supported by a separate study that arrived at the same conclusions.^[21]

In his 1867 book The English Constitution, Walter Bagehot praised parliamentary governments for producing serious debates, for allowing for a change in power without an election, and for allowing elections at any time. Bagehot considered the four-year election rule of the United States to be unnatural, as it can potentially allow a president who has disappointed the public with a dismal performance in the second year of his term to continue on until the end of his four-year term. Under a parliamentary system, a prime minister that has lost support in the middle of his term can be easily replaced by his own peers.

Although Bagehot praised parliamentary governments for allowing an election to take place at any time, the lack of a definite election calendar can be abused.

Previously under some systems, such as the British, a ruling party could schedule elections when it felt that it was likely to retain power, and so avoid elections at times of unpopularity.

(Election timing in the UK, however, is now partly fixed under the Fixed-term Parliaments Act 2011.) Thus, by a shrewd timing of elections, in a parliamentary system, a party can extend its rule for longer than is feasible in a functioning presidential system. This problem can be alleviated somewhat by setting fixed dates for parliamentary elections, as is the case in several of Australia's state parliaments. In other systems, such as the Dutch and the Belgian, the ruling party or coalition has some flexibility in determining the election date. Conversely, flexibility in the timing of parliamentary elections can avoid periods of legislative gridlock that can occur in a fixed period presidential system. In any case, voters ultimately have the power to choose whether to vote for the ruling party or someone else.