Understanding the role of institutions can help explain why gender equality policies often fail and why the efforts of gender equality advocates are frequently frustrated. Focusing on micro-foundations and using cases from comparative politics, the article builds a model that specifies the mechanisms whereby political institutions are systematically gendered. Political opportunities and outcomes are shaped not only by rules 'about gender' but also by seemingly neutral rules that have 'gendered effects', due to their interaction with institutions outside the realm of formal politics. Rules shape behaviour in gendered ways through mechanisms of regulation, obligation and persuasion. Actors reproduce gendered institutions through enacting rules, but they also generate change through adapting, resisting or reforming them. The article specifies concepts and methods for researching how political institutions are gendered within and across political systems. It also identifies points of intervention for those seeking to build more gender-just political institutions.