The chapter discusses Sartori’s theory of parties and party systems. While only a fragment of Sartori’s theory of political parties has survived, and it was not a theory that strayed much from the usual approaches to this subject at the time, his theory of party systems is impressive. It is one of the most original and respected things he ever wrote. This is especially true of his classification and typology of party systems, two distinct phenomena. His classification is based on one criterion (the format of a party system or the number of relevant parties in the system) and leads to the identification of classes. His typology is based on two criteria (the format and mechanics of a party system) and leads to identifying types. The mechanics of a party system take a different form in a democracy and a non-democracy. In a democracy, it is the ideological distance between parties; in a dictatorship, it is the intensity of ideology. Out of all types of party systems, polarised pluralism is the most important in terms of its functioning. It is one of the most remarkable contributions to analysing party systems in general. The chapter also analyses the complicated relationship between Sartori and the founder of contemporary research on parties and party systems, Maurice Duverger.