23 U.S. states have actively participated in the design and/or implementation of three regional cap-and trade programs to reduce greenhouse gas emissions: the Regional Greenhouse Gas Initiative (RGGI), the Midwestern Greenhouse Gas Reduction Accord (Midwestern Accord), and the Western Climate Initiative (WCI). Four Canadian provinces are also active participants. Five U.S. states, five Mexican states, and one Canadian province are observers of the WCI.

Together, the active U.S. state participants represent 51% of U.S. GHG emissions, 61% of U.S. GDP, and 67% of U.S. population.