The Special Report on Global Warming of 1.5°C identifies cities and urban areas as one of four critical global systems that can accelerate and upscale climate action, but recognizes this will require major transitions in how both mitigation and adaptation are undertaken. This C40 Summary for Urban Policymakers has been produced in discussion with city practitioners and policymakers, as well as leading city networks and non-governmental organizations. It seeks to provide urban stakeholders access to the most advanced science on why the transition to a 1.5°C world is necessary and how it can be achieved.