As it becomes clear that global warming is a reality, states are increasingly taking measures to regulate the emission of greenhouse gases such as carbon dioxide (CO2). These efforts come largely in response to the federal government’s failure to regulate CO2 emissions. Perhaps the most significant and novel example of states’ efforts to combat this problem is the Regional Greenhouse Gas Initiative (RGGI), a multi-state attempt to establish a regional cap-and-trade program targeting CO2 emissions produced by fossil fuel-fired power plants. RGGI faces significant obstacles in its path to full implementation, including the possibility that it violates the Compact Clause of Article I of the United States Constitution. This Note argues that in its current iteration, RGGI likely does not conflict with the federal government’s Compact Clause power as delineated by the Supreme Court in U.S. Steel Corp. v. Multistate Tax Commission. If RGGI’s administrative body is ultimately vested with greater regulatory and enforcement powers, however, this Note concludes that the outcome under U.S. Steel could be much different.