Several studies from northern temperate regions have shown that afforestation on poorly buffered soils and in areas of high atmospheric pollution and marine salt influence, can result in profound changes to surface water quality and to the ecology of aquatic systems. The preparation of land for planting and subsequent development of a large canopy of trees can also result in significant changes in water budgets, stream hydrographs and water yields, in comparison to unafforested moorland. Attention has focused on the geologically sensitive areas that are negatively affected; on well buffered soils less significant changes in stream chemistry, due to afforestation, may be expected. Extrapolation from abroad may not be universally appropriate, as in many parts of Ireland, the soils and geology are generally well buffered, atmospheric pollution (by European standards) is low and the ecology of the systems is different from that in other countries. This review outlines the nature of changes to freshwater systems that have been found in geologically sensitive areas, but extrapolation of results from such afforested areas to all areas under afforestation in general must be undertaken with care. -from Authors