[1] Steering committee members include: London, Freetown, Phoenix, Medellin, Copenhagen, Paris, Dhaka North, Nairobi, Tokyo, Rio de Janeiro, Montreal, Milan, Seoul, Oslo and Hong Kong.

[7] Working across multiple sectors and initiative areas, C40 convenes networks[8] of cities providing a suite of services in support of their efforts, including: direct technical assistance; facilitation of peer-to-peer exchange; and research, knowledge management & communications.

C40 is also positioning cities as a leading force for climate action around the world, defining and amplifying their call to national governments for greater support and autonomy in creating a sustainable future.

[9] C40 started in October 2005 when London Mayor Ken Livingstone convened representatives from 18 megacities to forge an agreement on cooperatively reducing climate pollution and created the 'C20'.

Serving as C40's first chair, Livingstone established the C40 Secretariat in London, set up the C40 Steering Committee, and initiated the use of C40 workshops to exchange best practices amongst participating cities.

Key milestones during his chairmanship include the full integration of the CCI Cities Program into the C40, and the C40 Mayors Summits in São Paulo and Johannesburg.

[18] The program brings together leading district-scale new-build and regeneration projects working to achieve "Climate Positive"—or net carbon negative—outcomes in cities around the world.

[25] Additional partners include:[26] C40's work is made possible by three strategic funders: Bloomberg Philanthropies, Children's Investment Fund Foundation and Realdania.