Around the world, urbanization and climate change are transforming societies and environments, and the stakes could not be higher for the poor and marginalized. The 2015 UN climate conference in Paris (COP-21) highlighted the need for coordinated action to address the profound injustice of the world’s most disadvantaged people bearing the greatest costs of climate impacts. Among those at the COP were mayors from around the world advocating for the important role of cities in these efforts.