International organisations (IOs) are central players in the architecture of global governance—from international peace and security to global economic governance, humanitarian assistance, and sustainable development. They draw on their expertise and the legitimacy they derive from their global or regional representativeness to facilitate international cooperation to address collective challenges. In this chapter, we examine the means and mechanisms through which IOs are governed. We begin with an introduction to IOs, focusing on the three foundational bodies that constitute the governance components of most IOs: member states, secretariats, and executive boards. We examine oversight and accountability mechanisms and consider four governance design features that vary across IOs: their mandate, financing, decision-making, and engagement with non-state actors. Finally, we reflect on some of the contemporary governance challenges IOs face today and describe different ways they are adapting and reforming to address some of the challenges.