After the gradual slowing down of the "third wave of democratization," electoral authoritarianism is rapidly emerging as a dominant form of contemporary autocracy. Political parties play a key role within the political and institutional structures of electoral autocracies. Pro-regime parties provide the dictatorial executive with electoral and legislative tools of sustaining power. At the same time, permitted opposition parties, while normally incapable of challenging the regime, are important for regime sustainability because they perform such vital functions as co-opting actual or potential opposition groups, facilitating power-sharing, and mobilizing electoral participation. The interactions among the dominant parties and the permitted opposition parties, if displaying sustainable cross-temporal patterns, constitute authoritarian party systems. Authoritarian Party Systems provides a theoretical discussion of electoral authoritarianism with special reference to authoritarian party systems; a methodological overview of party system research with special reference to the problems caused by the authoritarian nature of the observed party systems; a comprehensive cross-regional and historical overview of authoritarian party systems; a quantitative analysis of their structural characteristics, including fragmentation, party system format, volatility, and nationalization; and in-depth discussions of the political regime determinants of authoritarian party systems and of the interplay between party systems and other components of the authoritarian institutional order. Quantitative analysis has been performed on an original database comprising cases of party-structured authoritarian regimes between 1945-2019. This content of the book is illustrated by case studies drawn from across the spectrum of contemporary authoritarian regimes.