Two common types of legislature are those in which the executive and the legislative branches are clearly separated, as in the U.S. Congress, and those in which members of the executive branch are chosen from the legislative membership, as in the British Parliament. Respectively termed presidential and parliamentary systems, there are innumerable variations of the two forms. It should be noted that while popular assemblies of citizens, as in direct democracy, are often called legislatures, the term should properly be applied only to those assemblies that perform a representative function.

In its early history, the English Parliament, like the States-General of France and the diet of the Holy Roman Empire consisted of representatives chosen according to classes or estates (see estate, in constitutional law). Out of the estates arose the typical bicameral system, in which an upper house represented the nobility and clergy and a lower house represented the bourgeoisie. Although the upper house assemblies of many countries are still nonelective or hereditary, they are generally much weaker than the popularly elected lower house and carry out only minor functions. Those states with unicameral legislatures include Finland and Israel.

The Congress of the United States is bicameral, but rather than being rooted in societal class differences, it is based upon principles of federalism. The founders of the American republic, in order to assure acceptance of the Constitution, gave each state equal representation in the Senate, as a gesture to the smaller states, and made membership in the House of Representatives dependent upon population size, thereby favoring the larger states. Most of the American state legislatures are also bicameral.

While rules of law have always been a concern for society, the use of legislatures for their establishment is a relatively modern phenomenon. In earlier times, human laws were considered part of the universal natural law, discoverable through the use of reason rather than made by the declaration of the people. With the growth of belief in positive law, the increasing need in emerging modern society for adaptable law, and the decline of monarchial power, however, legislatures with law-making powers came about. One of the oldest legislatures (with the possible exception of Iceland's Althing and the Isle of Man's Tynwald) is the English Parliament, which, although originally nonelective and advisory to the king, has evolved over the centuries to the point where its lower house is now elected through universal suffrage and possesses the sovereign power of the state.

Some other modern national legislatures are the U.S. Congress, the Cortes (Spain), the Knesset (Israel), the Dáil Éireann (Ireland), the Bundestag (Germany), the Folketing (Denmark), the Riksdag (Sweden), the Storting (Norway), and the Congress of People's Deputies (Russia). The term parliament is often applied to national legislatures without regard to the official designation.