Wisdom in the development of public policy requires objective assessments of fact. Scientific understanding of acid precipitation and its biological consequences has increased substantially in recent years but needs to be expanded still further. At present much more is known about the nature and extent of effects of acid precipitation in aquatic than in terrestrial ecosystems. Increasing acidity of precipitation has been shown to induce extinction of fish and other major changes in the health and productivity of many different types of organisms in poorly buffered lakes in large parts of Europe and eastern North America. Thus, available knowledge is adequate to begin the process of formulating public policies to protect sensitive aquatic ecosystems from these injurious influences. By contrast, recent research on terrestrial ecosystems has shown that (1) atmospheric deposition contains both beneficial nutrients and injurious substances; (2) terrestrial plants, animals, and ecosystems vary greatly in susceptibility, tolerance, and adaptability to changes in atmospheric deposition; (3) injury is most likely when rapid changes in the chemical climate coincide with a vulnerable life form or life stage; (4) simulated acid rain can cause leaching of nutrients from foliage and from soil, and both direct and indirect injury to terrestrial vegetation.