This article explores institutional diffusion in international environmental governance, specifying the conditions under which an existing set of institutions provides a template for new institutions. Prior institutional experiences can help to resolve bargaining problems, reduce transaction costs and provide information about likely performance. The authors discuss five examples of institutional diffusion in international environmental affairs and outline some causal mechanisms and conditions that facilitate or block the diffusion of institutional characteristics. As a baseline analysis, founded on assumptions that abstract from politics, a functional argument is developed about the conditions under which mimetic diffusion, reflecting a pattern of imitation, can occur. Although the focus in this short article is on this functional argument, the authors recognize that state interests and power, ideology, and private interests also play significant roles in facilitating or inhibiting institutional diffusion in international environmental affairs.