How do changes in society that increase the heterogeneity of the citizenry shape democratic party systems? This book seeks to answer this question. To do so, it focuses on the key mechanism by which social heterogeneity shapes the number of political parties: new social groups successfully forming new, sectarian parties. Why are some groups successful at this while others fail? Drawing on cross-national statistical analyses and case studies of Sephardi and Russian immigration to Israel and African American enfranchisement in the United States, this book demonstrates that social heterogeneity does matter. However, it makes the case that to understand when and how social heterogeneity matters, factors besides the electoral system – most importantly, the regime type, the strategies played by existing parties, and the size and politicization of new social groups – must be taken into account. It also demonstrates that sectarian parties play an important role in securing descriptive representation for new groups.