The C40 has made assertive claims with respect to its ability to engender increased engagement, ambition, and scope of climate governance over both time and space. This chapter provides an independent corroboration of these claims, which have to-date been based on internal network data and analysis, by drawing on a novel dataset of over 10,000 climate governance actions adopted by C40 cities between 2001 and 2018. The chapter confirms that the C40 has increased the level of member city engagement, action, and ambition across geographic and economic divides. This renders the C40 distinct from other voluntary city-networks such as ICLEI and is deeply puzzling given the inability of these networks to deploy coercion or hard compliance mechanisms to close the gap between nominal commitments and concrete actions. The chapter concludes by considering three alternative explanations: as a function of economic development; as a function of inter-city learning, and as a function of the efforts of the network bureaucracy. Each is shown to be incomplete, thus demonstrating the need for a novel means of theorizing coordination in city-networks like the C40.