This paper uses network analysis to study the structural properties of international environmental cooperation. We investigate four pertinent hypotheses. First, we quantify how the growing popularity of environmental treaties since the early 1970s has led to the emergence of an environmental collaboration network and document how collaboration is accelerating. Second, we show how over time the network has become denser and more cohesive, and distances between countries have become shorter, facilitating more effective policy coordination and knowledge diffusion. Third, we find that the network, while global, has a noticeable European imprint: initially, the United Kingdom and more recently France and Germany have been the most important players to broker environmental cooperation. Fourth, international environmental coordination started with fisheries and the sea but is now most intense on waste and hazardous substances. The network of air and atmosphere treaties has distinctive topological features, lacks the hierarchical organization of other networks, and is the network most significantly shaped by UN-sponsored treaties.