Abstract: Research on acid deposition can be traced to the mid-19th century, but has expanded greatly in the last 50 yr, focussing in particular upon the effects on ecosystem of acid deposition caused by the combustion of fossil fuels. Observational studies first associated such acidification with it chemical and biological effects. They were supplemented in 1976 by experimental acidification of a whole lake. By 1980, paleoecological techniques were employed to investigate past histories of acidification. In the 1980s, critical loads were calculated to measure how much acid deposition could be tolerated by aquatic ecosystems. Reductions in emission of sulfur dioxide, as well as experimental studies in the mid-1980s, allowed investigations of recovery from acidification. At the same time, other studies showed that lake sediments and wetland peats mitigate acid deposition by reducing sulfate and nitrate. In the mid-1990s it became apparent that acid deposition interacts with other environmental stresses such as climate warming and ozone depletion.