This chapter advances a theoretical framework for studying how, why, and with what impact on audiences, global governance institutions (GGIs) are legitimated and delegitimated. The first component of the framework concerns the distinction between agents and objects. An agent enacts practices of (de)legitimation, whereas an object of (de)legitimation is what is being (de)legitimated, namely the GGI or specific policies. The second component is an agent-structure approach enabling the study of (de)legitimation processes within broader institutional and structural contexts. In this regard, the chapter identifies institutional set-up, policy field, and social structure as particularly relevant factors to account for variation in (de)legitimation across GGIs. The third component of the framework consists of practices, justifications, and audiences of (de)legitimation. Legitimation practices are what different agents engage in when they legitimate or delegitimate GGIs. Justifications are the substantive normative content these agents draw on when engaging in such practices. Audiences are the actors on the receiving end of these processes. The chapter develops overall theoretical expectations related to variation in practices, justifications, and audiences of (de)legitimation across GGIs.