A parliamentary system, or parliamentarism, is distinguished by the executive branch of government being dependent on the direct or indirect support of the parliament, often expressed through a vote of confidence. Hence, there is no clear-cut separation of powers between the legislative and executive branches of government. Parliamentary systems usually have a clear differentiation between the head of government and the head of state.

The term parliamentary system does not mean that a country is ruled by different parties in coalition with each other. Such multi-party arrangements are usually the product of a voting system known as proportional representation. Parliamentary nations that use first past the post voting usually have governments composed of one party. The United Kingdom, for instance, has not had a coalition government since World War II. However, parliamentary systems of continental Europe do use proportional representation, and it seems that PR voting systems and coalition governments usually go together.

The executive is typically a cabinet, and headed by a prime minister who is considered the head of government, but parliamentarism has also been practised with privy councils and the Senate of Finland. The prime minister and the ministers of the cabinet typically have their background in the parliament and may remain members thereof while serving in cabinet. The leader of the leading party, or group of parties, in the parliament is often appointed as the prime minister.

In many countries, the cabinet, or single members thereof, can be removed by the parliament through a vote of no confidence. In addition, the executive can often dissolve the parliament and call extra-ordinary elections.

Under the parliamentary system the roles of head of state and head of government are more or less separated. In most parliamentary systems, the head of state is primarily a ceremonial position, often a monarch or president, retaining duties that aren't politically divisive, such as appointments of civil service.

In many parliamentary systems, the head of state may have reserve powers which are usable in a crisis. In most cases however, such powers are (either by convention or by constitutional rule) only exercised upon the advice and approval of the head of government.

Parliamentary systems vary as to the degree to which they have a formal written constitution and the degree to which that constitution describes the day to day working of the government. They also vary as to the number of parties within the system and the dynamics between the parties. Also, relations between the central government and local governments vary in parliamentary systems, they may be federal or unitary states. Parliamentary systems also vary in the voting freedom allowed back bench legislators.

Parliamentarism is praised, relative to presidentialism, for its flexibility and responsiveness to the public. It is faulted for its tendency to sometimes lead to unstable governments, as in the German Weimar Republic, the French Fourth Republic, Italy, and Israel.

Parliamentarism may also be heeded for governance in local governments. An example is the city of Oslo, which has an executive council as part of a parliamentary system.

Types of parliamentary systems

The most geographically widespread parliamentary system is known as the Westminster System; named after Westminster in London, the meeting place of Britain's parliament. The Westminster system is used in Britain and in many member nations of the Commonwealth of Nations. In this model the head of state has considerable reserve powers which have been limited in practice by convention rather than explicit constitutional rule.

One major difference between the Westminster system and the types of parliamentarism used on the Continent and in non-Commonwealth monarchies outside of Europe is the voting system. Most Westminister systems use a kind of voting system, as mentioned above, known as "first past the post." In first past the post, each district elects one representative and that representative can be elected with a plurality. All Continental parliamentary systems use some kind proportional representation, usually the list system. First past the post favors a two-party system, proportional representation favors a multi-party system.

Other differences arise over the power of the cabinet relative to the rest of the assembly. France's Third and Fourth Republics were parliamentary, but most of the public policy impetus came from the assembly, not the cabinet - the so called "regime d'assemble." France's system was also disorganized compared to Britain's.

Several nations that are considered parliamentary actually have presidents who are elected separately from the legislature and who have certain real powers. Examples of this type of governance are Ireland and Austria. In both of these nations, there is a tradition for the president to not use his powers. France's Fifth Republic has a separately elected president who has a large role in government, but who is constitutionally weaker than presidents in Ireland and Austria.

France is considered to have a "Semi-presidential system" of government. Some scholars see France's government as half presidential, half parliamentary. Other scholars, like Arend Lipjhart, see France's system as alternating between presidentialism and parliamentarism. Lipjhart says that when the National Assembly is controlled by the party opposite the president - cohabitation, France is fundamentally parliamentary. When the president's party is in power in the National Assembly, the president, as party leader, is the dominant force in government.