Major emitting facilities are required to comply with BACT standards for each pollutant subject to regulation under the CAA. This requirement—initially thought to be inconsequential—has now become a dominant feature of the PSD program, for the first time subjecting greenhouse gas emissions from stationary sources to federally mandated pollution control standards. This Article is the fifth in a series on the CAA’s complex PSD program. The first four Articles, which appeared in the December 2005, January 2006, January 2008, and January 2011 issues of ELR News & Analysis, focused on baselines, increments and ceilings, on the permit triggers for new facilities, and on area classification and redesignation.