Approaches to the study of party systems are multiple, as are their definitions. In its simplest form, the party system is conceived of as a set of patterned relationships between political parties competing for power in a given political system. Such a notion assumes the existence of rules, norms, and regularities in party interactions, concerning mainly coalition-building efforts and electoral competition. This implies also that a party system is composed, as any other system, of distinguishable parts and the empirically testable quality of its “systemness.” Below, some major features of such systems, for example, the kinds and numbers of parties involved, their evolution and organization over time, their social bases and dynamic processes, and their relationship to institutional aspects of electoral systems and government formation, are discussed. An ambitious understanding of a system assumes that it displays features that do not belong to a single entity, that is, to one ...