The CAA's PSD program is extraordinarily complex. This Article is the fourth in a series on PSD under the CAA. The first three Articles, which appeared in the December 2005, January 2006, and January 2008 issues of News & Analysis, focused on baselines, increments, and ceilings, and on the permit triggers for new facilities. In this Article, Prof. John-Mark Stensvaag turns his focus to the area classification system controlling the degree of air quality deterioration permitted in specific areas of the United States, to the initial allocations of geographic areas by Congress among the various classes, and to the process by which states and Native American tribes may redesignate certain areas to allow greater or lesser degrees of degradation.