In December 2012, the United Nations General Assembly (UNGA) replaced the United Nations Environment Programme's Governing Council with the larger, universal membership United Nations Environment Assembly of the United Nations Environment Programme (UNEP). This major step for UNEP demonstrates the continuing strength of an idea that took root in the mid-19th century, namely the importance of assemblies of all member states as primary governing bodies in intergovernmental organizations (IGOs). This chapter provides an overview of these assemblies, their purposes, and workings. It also touches on the somewhat different phenomenon of assemblies of state parties to international treaties. It begins by looking at the antecedents of the concept of general assemblies.