The C40 city-network claims a position of global leadership in the governance of climate change. This chapter provides a brief overview of the history of the network, its member cities, and their collective aims and objectives. The chapter introduces the empirical puzzle around which the book is organized, namely the ability of the C40 to achieve coordinated action from a diverse collection of cities despite relying on voluntary participation and engagement. The ability to do so sets the C40 apart from other similar city-networks and begs the question as to how it has been able to achieve coordination and collective effort. The chapter asserts that such voluntary coordination is only possible through the formation of a collective identity and draws on ideas from the scholarship on social fields, social constructivism, and social movements to develop a theory of global urban governance fields that explains when, how, and why the C40 has managed to generate convergence around a set of governance norms and a shared governance identity.