This article proposes an analytical framework for exploring policy responses to common challenges of environmental governance. Observing that governance involves multiple processes, I begin by identifying a conceptual platform for studying unilateral learning and adaptation as well as international cooperation as integral and interacting components of a complex governance system. I propose the concept of co-evolution as the cornerstone of this platform and distinguish between two modes of co-evolution: diffusion and cooperation. The article draws findings and propositions from recent literature to identify the mechanisms at work and the conditions under which they foster mutually beneficial solutions. Indicating how important governance challenges differ with respect to these conditions, I build the case for a diagnostic and differential approach that matches capacity-building and policy strategies with the challenge in focus.