The Climate Opportunity report investigates the economic, social and environmental benefits for climate change mitigation action in cities. Enhanced policies and measures for residential building retrofits, bus networks and district-scale renewable energy can generate millions of jobs, save billions of dollars for households, and prevent hundreds of thousands of deaths related to ambient air pollution worldwide. The analysis is based on the development of new methodologies, which provide local and national policymakers with a blueprint for how cities and nations can evaluate these impacts and benefits to develop their own robust cases for climate change mitigation policies.