As the populations in urban centers rise, so too does their influence on the world stage. Cities across the globe are recognizing their power and potential as nodes in global networks of social, economic, and cultural influence, looking to each other to collaborate and learn. Bypassing national government as traditional facilitators of such interactions, city diplomacy has the capacity to strengthen bonds between localities that can result in economic benefits for both. It rests on the idea and reality that cities are innovation ecosystems. From Boston to Houston to Portland, cities have the material resources, human capital, and the vibrant institutions to use both to generate economic growth. Recognizing their rising importance, coordinated efforts such as C40 Cities Climate Leadership Group (C40), ICLEI – Local Governments for Sustainability (ICLEI), the United Cities and Local Governments (UCLG), and the Compact of Mayors are at the fore of harnessing the power of cities to advance international cooperation.