Recent decades have shown that political parties differ in their ability to change their strategies and structures in response to a changed environment. This variation in party adaptation is important because the decline of old parties has often made room for radical and populist parties to rise and recently caused widespread concern over the future of liberal democracy. Focusing on three center-right parties in Germany, Italy, and Austria, this book shows that party factions, defined as organized internal groups with weak formal ties to the central party, can support party adaptation by integrating competing actors and helping new leaders to rise and new ideas to circulate. Too many factions, however, risk paralyzing decision-making by leading to a proliferation of competing groups and cycling majorities. The relationship between factionalism and party adaptation is thus inverted U-shaped. The book takes a comparative-historical perspective to explain the emergence and development of different levels of factionalism and their effect on party adaptation, highlighting the long-term impact of parties’ early organizational choices. Empirically, it builds on extensive archival research and additional shadow cases from Argentina, France, and Japan. Overall, the book provides evidence on the varying organization, factionalism, and adaptation of six major incumbent parties and covers more than 75 years.