The 1990 U.S. Clean Air Act Amendments (CAAA) instituted a national program in tradable sulfur dioxide (SO2) emission permits, referred to as "emission allowances," in the U.S. electricity sector. This paper provides a survey and assessment of the SO2 allowance trading program with a focus on the role of innovation. Over the last decade the cost of compliance has fallen dramatically compared with most expectations, and today the total cost of the program is 40-140% lower than projections (depending on the timing of those projections and the counter-factual baseline considered). Marginal costs of reductions are less than one-half the cost considered in most analyses at the time the program was introduced. Innovation accounts for a large portion of these cost savings, but not as typically formulated in economic models of research and development (R&D) efforts to obtain patent discoveries. Innovation under the SO2 allowance trading program involves organizational innovation at the firm, market and regulatory level and process innovation by electricity generators and upstream fuel suppliers. An important portion of the cost reductions that are evident was already in the works prior to and independent of the program. Nonetheless, the allowance trading program deserves significant credit for providing the incentive and flexibility to accelerate and to fully realize exogenous technical changes that were occurring in the industry. This marks a significant departure from conventional approaches to environmental regulation, which would not be expected to capture these savings. The ongoing transition to restructuring of electricity markets and expanding competition in electricity generation complements the design of the SO2 allowance trading program by providing firms with full incentives to reduce costs of pollution control.