Does the United States need the Clean Air Act’s Prevention of Significant Deterioration (PSD) program to guard against degradation of air quality? In their recent Comment in these pages, John C. Evans and Donald van der Vaart say no, but the right answer is more nuanced. While the program is flawed in some respects, PSD helps to protect national parklands, guard against pollution “hot spots,” and decrease air pollution emission levels in the United States, thus helping to remedy regional pollution problems.