Matthew J. Hoffmann explores the fundamental question of who should participate in the global response to ozone depletion and climate change. Blending social constructivist theory with insights from the study of complex adaptive systems, Hoffmann develops a unique framework for understanding the emergence and evolution of participation norms, which define the appropriate global response and shape how states have perceived the problems, defined their interests and strategies, and pursued governance. The explanation is rigorously developed through an innovative combination of formal analysis and in-depth empirical case studies. Agent-based computer simulation modeling is employed to explore essential norm dynamics, analysis that is complemented and extended by process-tracing case studies that examine governance activities from 1986 through 2003. The result provides the understanding necessary for improving global responses to environmental problems.