Understanding contemporary global governance requires a focus on informality. States increasingly govern through informal intergovernmental organizations, transnational public–private governance initiatives, and other informal institutions. Even within formal institutions, informal practices complement or override formal rules. And diverse informal groupings operate in the orbit of governance institutions, framing novel issues and placing them on policy agendas. We address these three aspects of informality – of, inside, and around global governance institutions. We first trace the nature and extent of the shift toward informal governance. We then consider a range of factors that may be driving the shift, drawing on major streams of International Relations (IR) theory; we treat these as candidate explanatory variables. Finally, we summarize the findings on those variables, and other theoretical insights, from the empirical chapters of this volume.