In recent decades a great expansion has occurred in world environmental organization, both governmental and nongovernmental, along with an explosion of worldwide discourse and communication about environmental problems. All of this constitutes a world environmental regime. Using the term regime a little more broadly than usual, we define world environmental regime as a partially integrated collection of world-level organizations, understandings, and assumptions that specify the relationship of human society to nature. The rise of an environmental regime has accompanied greatly expanded organization and activity in many sectors of global society. Explaining the growth of the environmental regime, however, poses some problems. The interests and powers of the dominant actors in world society—nation-states and economic interests—came late to the environmental scene. Thus these forces cannot easily be used to explain the rise of world mobilization around the environment, in contrast with other sectors of global society (for example, the international economic and national security regimes).