This chapter describes the theoretical framework of the book. It starts by discussing the relevance of intra-party politics in light of recent real-world examples of factional fights and splits concerning France, Germany and Italy. Then it provides an analytical summary of the existing literature on intra-party politics and factionalism. Finally, based on the comparison between parties with directly or indirectly elected party leaders, it presents a game–theoretic model to investigate party unity and party fission by analyzing the interplay between the party leader and a minority faction. According to the possible solutions of the game and to the parameters discussed therein, the chapter raises a number of general expectations (that will be tested in the rest of the book) related to the degree of factionalism, the allocation of policy or office payoffs. And the occurrence of party splits.