In 1972, Congress enacted the Federal Water Pollution Control Act, commonly known as the Clean Water Act (CWA), to 'restore and maintain the chemical, physical, and biological integrity of the Nation's waters.' Subsequently amended in 1977 and reauthorized in 1987, the CWA has generally been credited with reversing a century-long trend in water quality degradation. To achieve these results, however, the Act has imposed certain costs on society. This study assesses the magnitude of these costs in recent years, now that the CWA has been in place for over a quarter century. To properly account for these costs, it is important to stress that some of the costs that are currently being incurred to reduce water pollution would have existed even without the CWA. Although the CWA established a discharge permit system and provided significant federal financial assistance for constructing public treatment facilities, significant public and private investments in water pollution control existed prior to 1972. These expenditures would most likely have continued to grow after 1972 even if the CWA had not been enacted. Therefore, the approach taken in this study is to, not only estimate recent nationwide costs of water pollution abatement (WPA), but also to simulate and deduct from these estimates the WPA costs that would have been incurred without the CWA.