Huseyn Aliyev examines how, when, and under which conditions democratic institutional reforms affect informal institutions in hybrid regimes, or countries transitioning to democracy. He analyzes the impact of institutional changes on the use of informal practices and what happens when democratic reforms succeed. Does informality disappear, or do elites and populations continue relying on informal structures? When Informal Institutions Change engages with a growing body of literature on informal practices and institutions in political science, economics, sociology, and beyond. Aliyev proposes expanding the analysis of the impact of institutional reforms on informal institutions beyond disciplinary boundaries, and combines theoretical insights from comparative politics with economic and social theories on informal relations. In addition, Aliyev offers insights that are relevant to democratization, institutionalism, and human geography. Detailed case studies of three transitional post-Soviet regimes—Georgia, Moldova, and Ukraine—illustrate the contentious relationship between democratic institutional reforms and informality in the broader post-Soviet context. Aliyev shows that in order for institutional reform to succeed in strengthening, democratizing, and formalizing institutions, it is important to approach informal practices and institutions as instrumental for its effectiveness. These findings have implications not only for hybrid regimes, but also for other post-Soviet or post-communist countries.