Air pollution and its effects on various ecosystems has been a subject of increasing controversy. The European forest damage situation has heightened the public's awareness of the problem. Historically, the observed pollution damage was believed to be due mainly to sulphur dioxide pollution. Recent research has indicated that other factors including nitrogen oxides, ozone and other photo-oxidants, and various climatic conditions contribute to the problem. The possibility of a multiple-pollutant effect is recognized and is leading to a more multi-disciplinary approach to the study of the effects of air pollution on forest ecosystems. In Canada, research has centred around the higher emission areas in the eastern part of the country. In the western provinces, there is a concern that the forests may be adversely affected in the long term. Alberta is in a fortunate situation because most of its soils are underlain by calcareous bedrock and can, therefore, neutralize in-coming acids more efficiently than areas covered by the Canadian Shield. However, sensitive areas do exist, and the long-term impact of continuous inputs into the system is not yet fully understood.