There is today a global recognition that we live in an ‘urban age’ of near-planetary urbanization where cities are at the forefront of all sorts of agendas. Yet little attention is offered to the active role of cities as political drivers of the urban age. There might today be more than two hundred ‘city networks’ globally, with thousands of para-diplomatic connections actively defining relations between cities, international organization and corporate actors. This actively networked texture of the urban age shapes all areas of policy and, not least, international relations, and holds much promise as to possible urban solutions to global challenges. Based on an overview of a representative subset of this mass of city-to-city cooperation (n=170), this article illustrates the landscape of city networking, its issue areas and institutional shapes, and its critical features. As we argue, city networks today are faced by a crucial challenge: while trying to overcome state-centric ‘gridlocks’ cities are, at the same time, building both political–economic as well as very material ‘lock-ins’. We need to pay serious attention to this impact of city diplomacy in international affairs, developing a greater appreciation of the path dependencies and responsibilities this diplomatic activity purports.