Cap-and-trade programs have become widely accepted for the control of conventional air pollution in the United States. However, there is still no political consensus to use these programs to address greenhouse gases. Meanwhile, in the wake of the success of the U.S. SO2 and NOx trading programs, private companies, state governments, and the European Union are developing new trading programs or other initiatives that may set precedents for a future national U.S. greenhouse gas trading scheme. This paper summarizes the literature on the "lessons learned" from the SO2 trading program for greenhouse gas trading, including lessons about the potential differences in design that may be necessary because of the different sources, science, mitigation options, and economics inherent in greenhouse gases. The paper discusses how the programs and initiatives mentioned above have been shaped by lessons from past trading programs and whether they are making changes to the SO2 model to address greenhouse gases. Finally, the paper concludes with an assessment of the implications of these initiatives for a future U.S. national greenhouse gas trading program.