Bicameralism is a type of legislature that is divided into two separate assemblies, chambers, or houses, known as a bicameral legislature. Bicameralism is distinguished from unicameralism, in which all members deliberate and vote as a single group. As of 2022^[update], roughly 40% of the world's national legislatures are bicameral, while unicameralism represents 60% nationally and much more at the subnational level.

Often, the members of the two chambers are elected or selected by different methods, which vary from jurisdiction to jurisdiction. This can often lead to the two chambers having very different compositions of members.

Enactment of primary legislation often requires a concurrent majority—the approval of a majority of members in each of the chambers of the legislature. When this is the case, the legislature may be called an example of perfect bicameralism. However, in many parliamentary and semi-presidential systems, the house to which the executive is responsible (e.g. House of Commons of the UK and National Assembly of France) can overrule the other house (e.g. House of Lords of the UK and Senate of France) and may be regarded as an example of imperfect bicameralism. Some legislatures lie in between these two positions, with one house able to overrule the other only under certain circumstances.

The British Parliament is often referred to as the "Mother of Parliaments" (in fact a misquotation of John Bright, who remarked in 1865 that "England is the Mother of Parliaments") because the British Parliament has been the model for most other parliamentary systems, and its Acts have created many other parliaments. The origins of British bicameralism can be traced to 1341, when the Commons met separately from the nobility and clergy for the first time, creating what was effectively an Upper Chamber and a Lower Chamber, with the knights and burgesses sitting in the latter. This Upper Chamber became known as the House of Lords from 1544 onward, and the Lower Chamber became known as the House of Commons, collectively known as the Houses of Parliament.

The Founding Fathers of the United States also favoured a bicameral legislature. The idea was to have the Senate be wealthier and wiser. Benjamin Rush saw this though, and noted that "this type of dominion is almost always connected with opulence". The Senate was created to be a stabilising force, not elected by mass electors, but selected by the State legislators. Senators would be more knowledgeable and more deliberate—a sort of republican nobility—and a counter to what James Madison saw as the "fickleness and passion" that could absorb the House.

He noted further that "The use of the Senate is to consist in its proceeding with more coolness, with more system and with more wisdom, than the popular branch." Madison's argument led the Framers to grant the Senate prerogatives in foreign policy, an area where steadiness, discretion, and caution were deemed especially important. State legislators chose the Senate, and senators had to possess significant property to be deemed worthy and sensible enough for the position. In 1913, the 17th amendment passed, which mandated choosing Senators by popular vote rather than State legislatures.

As part of the Great Compromise, the Founding Fathers invented a new rationale for bicameralism in which the Senate had an equal number of delegates per state, and the House had representatives by relative populations.

Many monarchies then followed them to some degree emulated the British "three-tier" model.