Twenty-three U.S. states and four Canadian provinces are actively participating in the design and implementation of three regional cap-and-trade programs to reduce greenhouse gas emissions (the Regional Greenhouse Gas Initiative - RGGI; the Midwestern Greenhouse Gas Accord; and the Western Climate Initiative - WCI). Participating U.S. states account for one-half of the U.S. population and Gross Domestic Product (GDP), and one-third of all U.S. greenhouse gas emissions. The Canadian provinces account for more than three-quarters of the Canadian population and GDP, and nearly one-half of Canadian GHG emissions. These efforts are formally observed by another 14 states and provinces across the United States, Canada, and Mexico.