Title IV of the 1990 amendments to the Clean Air Act initiated a historic experiment in incentive-based environmental regulation through the use of tradable allowances for emission of sulfur dioxide by electric generating facilities. To date, relatively little allowance trading has taken place; however, the costs of compliance have been much less than anticipated. The purpose of this paper is to address the apparent paradox that the allowance trading program may not require (very much) trading to be successful. Title IV represented two great steps forward in environmental regulation: first a move toward performance standards and second formal allowance trading. The first step has been sufficient to date for improving dynamic efficiency and achieving relative cost-effectiveness.