This chapter deals with political parties, focusing on why they emerged, how they can be classified, what functions they perform, and how they interact. It identifies two phases in the development of political parties. The first parties were intended to structure the work of legislatures, and later evolved into mass parties to structure the votes of electors, catch-all parties to win more votes irrespective of ideological appeal, and cartel parties more dominated by party professionals. The chapter also considers seven functions typically carried out by a political party, irrespective of whether they operate in democracies or authoritarian regimes: legitimation of the political system, integration and mobilization of citizens, representation, structuring the popular vote, aggregation of diverse interests, recruitment of leaders for public office, and formulation of public policy. Finally, it discusses various types of party outside the West, party systems, and some of the challenges facing political parties today.