Unicameralism is the practice of having only one legislative or parliamentary chamber. Many countries with unicameral legislatures are often small and homogeneous unitary states and consider an upper house or second chamber unnecessary.

Unicameralists claim that an upper house makes no sense in a democracy, saying that if an upper house is democratic, it simply mirrors the equally democratic lower house, and is therefore unnecessary. They argue that the functions of a second chamber, such as reviewing or revising legislation, can be performed by parliamentary committees, while further constitutional safeguards can be provided by a written Constitution.

In many instances these states had a second chamber and subsequently abolished it. This is either because an elected upper house had duplicated the lower house and obstructed the passing of legislation, like the Landsting in Denmark (abolished in 1953), or because an appointed chamber had proven ineffectual, like the Legislative Council in New Zealand (abolished in 1951).

Examples of single chamber parliaments or legislatures

• National People's Congress in the People's Republic of China
• Legislative Yuan of the Republic of China (Taiwan)
• Croatian Sabor
• The Folketing of Denmark
• Parlamento Nacional of East Timor
• The Eduskunta of Finland
• The Vouli ton Ellinon of Greece
• The Althing of Iceland
• Knesset of Israel
• National Assembly of Iraq
• New Zealand House of Representatives
• The Storting of Norway (may divide into two chambers for some purposes)
• Assembly of the Republic of Portugal
• Assemblée Nationale of Seychelles
• Kukhoe of South Korea
• Parliament of Singapore
• Parliament of Sri Lanka
• The Riksdag of Sweden (since 1971)
• Turkey's Büyük Millet Meclisi
• Asamblea Nacional of Venezuela

Some of the subnational entities with unicameral legislatures include Nebraska in the United States, Queensland in Australia, all of the provinces and territories in Canada, and all of the German Bundesländer (Bavaria having dropped bicameralism in 1999).

In the United Kingdom, the devolved Scottish Parliament, National Assembly for Wales and Northern Ireland Assembly are also unicameral.

Virtually all city legislatures are also unicameral in the sense that the city councils aren't divided into two chambers. Until the turn of the 20th century, bicameral city councils were common in the United States.

See also: Bicameralism, Tricameralism, List of national legislatures