Legislatures are a key institution in modern political systems. They control the executive: in parliamentary systems, they elect the government, and in presidential systems, they often approve the members of the cabinet who were appointed by the president. Moreover, legislatures and their committees are the main bodies in a political system in which new policies - in the form of law proposals - are discussed, designed and decided upon. In this chapter, we provide an overview of the current research on the role of legislatures in modern democracies, focusing mainly on legislative behaviour. Starting from the principal-agent framework, we present the main findings on legislative behaviour by focusing on the analysis of legislative voting and legislative speech. Lastly, we discuss possible avenues for future research on legislatures and legislative behaviour.