For nearly thirty years, the Clean Air Act’s (CAA) New Source Review (NSR) program, as applied to existing industrial facilities, was a backwater of environmental regulation. As its name suggests, Congress designed this program to ensure that major new sources of air pollution, such as factories and power plants, install the latest pollution control devices when they are built. Congress did not impose these same requirements on existing facilities because it believed that the costs, and associated disruption, were not justified by the projected environmental benefit to be gained. Rather, it determined that existing plants would have to go through the NSR process, and install additional pollution controls, only if they undertook projects that increased their actual capacity to emit covered air pollutants. This, at any rate, was the understanding of both EPA and industry, and the basis on which EPA’s NSR regulations were adopted.