In 2015, world leaders issued a clarion call to promote sustainable development by tackling climate change and environmental sustainability, growing inequalities and social exclusion, and ensuring economic opportunities for all. To this end they adopted Agenda 2030 and its 17 Sustainable Development Goals (SDGs). In three years, strong progress has been made by a large number of countries, but there is also growing interest and commitment from local governments. Thousands of cities worldwide are putting sustainable development at the top of their agenda, recognizing that economic prosperity, social inclusion, and environmental sustainability are foundations of urban development. They use Agenda 2030 and its associated SDGs as shared goals and metrics to track progress, enable city-to-city learning, and as a means to raise domestic and international resources.