This study tests the assumption that political regimes have uniform structures, which is central in the research on political regimes. The various analyses undertaken in the study are based on the concept of regime heterogeneity, which refers to the degree of institutional variation in political regimes. This concept is introduced in this book and opens the way to the possibility that political regimes may be more or less uniform in their structure. Furthermore, the study provides empirical support for the alternative assumption that political regimes may have institutional variations in their structure. The study also illustrates how politics functions when a political regime is characterized by regime heterogeneity. This book provides conceptual and empirical contributions that strengthen our understanding of how political regimes are constructed, but also how political systems function when political regimes have institutional variation.