Informal modes of cooperation are a central element of the complex institutional architecture of contemporary global governance. Collectively and individually, the contributions to this special issue broaden the emerging research on informal governance in world politics and provide novel empirical analyses based on unique data. In this introduction, we outline the research questions and puzzles that the special issue addresses. We then sketch three types of informality in world politics: Informality of institutions, within institutions, and around institutions. We discuss each type and provide examples from the contributions to the special issue and the existing literature. We consider how differentiating among these types of informality provides novel insights into the causes of informal global governance. We also identify candidate independent variables which, individually and in combination, should allow researchers to explain the striking variation in the growth and distribution of informal governance in world affairs. We summarize the main findings of the contributions and conclude by outlining an agenda for future research on informal governance in world politics.