Under title IV of the Clean Air Act, Congress authorized the U.S. Environmental Protection Agency (EPA) to establish the Acid Rain Program. The principal goal of the program is to achieve significant environmental benefits through reductions in sulfur dioxide (SO<INF>2) and nitrogen oxide (NO<INF>X) emissions, the primary components of acid rain. Acid rain causes surface water acidification, damages trees at high elevations and accelerates the decay of building materials. In addition, air concentrations of SO<INF>2 and NO<INF>X degrade visibility in large parts of the country and acidic aerosols derived from these emissions may pose a risk to public health. The Acid Rain Program departs from traditional regulatory methods by introducing an SO<INF>2 allowance trading system that lowers the cost of reducing emissions by allowing electric utilities as a group to seek out the least costly methods of control. Utility units affected under title IV are allocated allowances based on their historic emissions and these units may trade allowances, provided that at the end of each year, each unit holds enough allowances to cover its annual SO<INF>2 emissions. Today's action establishes an additional component to the Acid Rain Program called the Opt-in Program. The Opt-in Program allows sources not required to participate in the Acid Rain Program the opportunity to participate on a voluntary basis. Such sources, known as combustion sources, would include small utility units and industrial boilers. These rules detail how combustion sources participate in the allowance market by ``opting in'' to the Acid Rain Program, as provided under section 410 of the Act. Congress envisioned the Opt-in Program as a means of generating additional allowances and through which the compliance costs of acid rain control in the utility sector could be reduced, while still meeting overall emissions reductions goals.