Mainstream comparative research on political institutions focuses primarily on formal rules. Yet in many contexts, informal institutions, ranging from bureaucratic and legislative norms to clientelism and patrimonialism, shape even more strongly political behavior and outcomes. Scholars who fail to consider these informal rules of the game risk missing many of the most important incentives and constraints that underlie political behavior. In this article we develop a framework for studying informal institutions and integrating them into comparative institutional analysis. The framework is based on a typology of four patterns of formal-informal institutional interaction: complementary, accommodating, competing, and substitutive.We then explore two issues largely ignored in the literature on this subject: the reasons and mechanisms behind the emergence of informal institutions, and the nature of their stability and change. Finally, we consider challenges in research on informal institutions, including issues of identification, measurement, and comparison.