The agreement reached at the 21st Conference of the Parties (COP21) in Paris could prove to be a historic turning point for reversing the currently unsustainable trends in the global energy system, provided that this heightened low-carbon ambition is translated into fast, radical and effective policy action. Even in the context of low fossil fuel prices, policy support for low-carbon technologies should mobilise alllevers available to accelerate research, development, demonstration and deployment (RDD&D) to make decarbonisation the preferred development path. Chief among such levers is governments support for urban energy transitions, a conclusion that is supported by the analysis of Energy Technology Perspectives 2016 (ETP 2016), which shows the vast number and size of cost-effective, sustainable energy opportunities available in cities. Realising this potential, and the multiple non-climate benefits it presents, will require national and local governments to work together effectively