Editors' Summary: The CAA's PSD program is extraordinarily complex. This Article is the third in a series on preventing significant deterioration under the CAA. The first two Articles, which appeared in the December 2005 and January 2006 issues of News & Analysis, focused on baselines, increments, and ceilings. In this Article, Prof. John-Mark Stensvaag turns his focus to the circumstances under which a new stationary source must obtain a PSD permit. After explaining the benefits of avoiding a PSD permit requirement, he explores the statutory and regulatory language relevant to construction of a new major emitting facility.