Today, more than half of the world's population – 4.2 billion inhabitants – live in cities. Cities are also responsible for 70% of CO2 emissions globally. Cities and their citizens must be at the forefront of combating climate change and the spread of epidemics. Mayors and government officials have now the opportunity to lead the urgent ecological transition so much needed and transform the cities. Now more than ever, our cities need a bold vision that is backed by action.