In Alaska Department of Environmental Conservation (ADEC) v. U.S. Environmental Protection Agency, the U.S. Supreme Court narrowly upheld orders issued by the U.S. Environmental Protection Agency (EPA) pursuant to §§113(a)(5) and 167 of the Clean Air Act (CAA or Act), prohibiting construction of a new power generator unit at a mine in Northwest Alaska. EPA issued the orders because it concluded that the Alaska Department of Environmental Conservation (ADEC), the state agency responsible for administering the CAA's prevention of significant deterioration (PSD) program in Alaska, had arbitrarily failed to require the mine operator to adopt the best available control technology (BACT) to limit emissions of nitrogen oxides (NOx) from the new generator unit. The state agency had argued that it had complied with the CAA's BACT provisions, and that the Act did not give EPA the authority to second-guess a state agency's decision as to what sort of pollution control equipment represents BACT. As it reached the Court, the controversy centered on whether EPA possessed the authority to reject a state agency's designation of BACT under the PSD program. What follows are excerpts of the brief of a group of amicus curiae--Environmental Defense, the National Parks Conservation Association, the Northern Alaska Environmental Center, and Alaska Community Action on Toxics--in support of respondent EPA. After the excerpt are some comments on the Court's opinion in ADEC and its impact.