Legislatures are political bodies essential to democracy and the rule of law. They present social scientists with numerous intriguing puzzles, with far-reaching implications for our understanding of political institutions. Why, and how, have these ancient assemblies, established in pre-democratic times, survived the transition to mass democracies? How have they adapted? How do they structure such processes as budgeting, legislation, and executive oversight? How do their members get selected, and what consequences flow from differences in these rules? What roles do committees and political parties play in contemporary legislatures?