Legislative committees are internal subunits of the legislature comprised of legislators and enjoying certain delegated authority. As a common form of legislative organization, committees play an important role in the functioning of the legislature, for example by influencing the content of legislation and holding the executive accountable. This article discusses the four most prominent theories that explain why committees are fundamental to the operation and everyday life of the US House of Representatives and the Senate: distributional theory, informational theory, cartel-party theory, and bicameral-conflict theory. It also considers attempts to test empirically theories of committees in legislatures outside the United States and examines comparative theories of legislative organization.