This chapter unpacks internal dynamics of political parties and introduces a factional theory of party behavior. The central assertion is that political parties are factional coalitions with different perspectives on political issues. The aim herein is twofold. First, the chapter traces how individual preferences aggregate into group preferences in the form of factional politics. Second, it explores the conditions under which one faction prevails over others. This chapter thus offers a theory of coalition-building within political parties by identifying different types of incentive structures and organizational resources. Factions strive for control over such resources to capture and control the party. Once they form a dominant coalition, they align the party’s trajectory with their own worldview. This chapter builds on the existing studies of behavioral and ideological change to address the issue of aggregation and fill the gap in the literature. It concludes with a brief discussion of these dynamics in the AKP, the Muslim Brotherhood, and Ennahda.