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 formalinformal 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. ver the last two decades, institutional analysis has become a central focus in comparative politics. Fueled by a wave of institutional change in the developing and postcommunist worlds, scholars from diverse research traditions have studied how constitutional design, electoral systems, and other formal institutional arrangements affect political and economic outcomes.1 These studies have produced important theoretical advances. Nevertheless, a growing body of research on Latin America,2 postcommunist Eurasia,3 Africa,4 and Asia5 suggests

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Comparative research on political institutions has begun to turn from issues of formal institutional design to issues of institutional strength. Rather than assuming a tight fit between formal rules and political behavior, these studies examine how variation in the stability and/or enforcement of formal rules shapes actors' expectations and behavior. This article explores the emerging research agenda on institutional strength. It disaggregates the concept of institutional strength into two dimensions-enforcement and stability-and it argues that institutions vary widely on both dimensions. The article then examines the sources of this variation and its implications for comparative research. It shows how recent research on weak institutions may be used to refine existing theories of institutional effects, design, and development, which should broaden the comparative scope of these theories. The conclusion examines ways of developing comparative measures of institutional strength.