The volume of NOx and SO2 emissions each year from unregulated, grandfathered power plants demonstrates that the goal of the CAA’s PSD program, to ensure that air quality standards under NAAQS do not in effect become a ceiling, has achieved only limited success. One significant challenge under the PSD program is the difficulty associated with identifying major emitting facilities that have made major modifications. This repeatedly results in statute-of-limitations problems for enforcement efforts. To resolve this enforcement difficulty, reviewing courts have split into two competing interpretations of the statutory and regulatory requirements of the PSD program. The proper interpretation of the CAA and EPA’s regulations demands that PSD requirements be interpreted to impose ongoing operational obligations.