The aim of this Bulletin is to summarise the current information available on the interactions between air pollution and forests. While it is primarily concerned with Great Britain, air pollution is an international problem, so information from other countries has been included. Pollution from point sources, such as aluminium smelters and brickworks, has not been included, and this paper deals mainly with long-range pollution and its possible regional-scale effects. A technical account of the formation of atmospheric pollutants is given. This is followed by a more general discussion of acidification and its effects on the environment. The symptoms shown by trees involved in the forest decline seen on the continent are described together with data from various national surveys; a major part of the report is concerned with the possible causes of this decline and its economic implications. Finally, research being undertaken by the Forestry Commission into the possible effects of air pollution on trees is described.