Much has been written about the economic and environmental performance of U.S. emissions trading programs for "acid rain" (sulfur dioxide) and nitrogen oxides. Less explored have been the unique roles and interactions of environmental regulators and the companies they regulate. I first examine how these roles change the way that regulators and companies operate within their own organizations and with each other. Next, I use examples from U.S. trading programs to illustrate the design and administrative features that allow program administrators and industry to best fulfill their respective roles. Finally, I examine briefly whether these features are present in the EU Emissions Trading System and determine the implications for its effectiveness.