Acid precipitation is a significant air pollution problem in North America and Europe. Here, large amounts of sulfur and nitrogen oxides and many other substances are emitted into the atmosphere from the combustion of fossil fuels, the smelting of sulfide metal ores, and other industrial processes. These oxides of sulfur and nitrogen are transported by wind over short and long distances and are transformed into a variety of substances that include sulfuric, nitric, and hydrochloric acids. These substances return to the surface of the earth in the form of acidic gases, aerosols, and precipitation. The resulting wet and dry deposition of acids and other chemical substances from the atmosphere is having a variety of effects on vegetation, soils, and surface waters. The purpose of this chapter is to review and evaluate the present state of scientific knowledge about the known and possible effects of acidic substances from the atmosphere on plants.