This chapter examines how competition between political parties gives rise to different party systems. In liberal democracies, competition for power is based on popular votes. The shape and dynamics of party systems are determined by the electoral game, with parties as main actors. A party system is thus essentially the result of competitive interactions between parties. A party system has three main elements: which parties exist, how many parties exist and how big they are, and how parties behave. An obvious but important point is that party systems must be composed of more than one political party. The chapter begins with a discussion of the origins of party systems, followed by an analysis of the format of party systems, such as two-party systems and multiparty systems. It then considers the influence of the electoral system on party systems, before concluding with an assessment of the dynamics of party systems.