Recent reviews of available data indicate that precipitation in a large region of North America is highly acidic when its pH is compared with the expected pH value of 5.65 for pure rain water in equilibrium with CO2. A growing body of evidence suggests that acid rain is responsible for substantial adverse effects on the public welfare. Such effects, which are discussed in this report, include the acidification of lakes and rivers, with resultant damage to fish and other components of aquatic ecosystems; acidification and demineralization of soils; possible reductions in crop and forest productivity, and deterioration of manmade materials.