As a base for political decisions a literature research on the influence of forests on the quality of drinking water was carried out. In Switzerland groundwater infiltrating through forests normally is of high quality for two reasons. Because of legal restrictions in forested areas little activities take place that have a negative impact on water quality (passive influence of the forest). Forests in general and especially forest soils alter or absorb chemical and biological pollutants (active influence of the forest). In most cases the passive influence is more important. The capability of forests to absorb pollutants is limited. Therefore negative influence of continuing Nitrogen input by aerial pollution on drinking water quality has to be expected.A quantification of this effect is not possible because the capability of a forest to absorb or alter pollutants depends on many factors. E.g. soils below beech trees absorb and alter more pollutants that soils below spruce. Forest management can have a positive long term influence on the active influence of forests on the water quality through altering the species of a forest and thus initiating a change in soil properties. Forest management may have a negative short term influence on water quality through harvesting. Clear cutting breaks open the nutrient cycle of the forest ecosystem on a large area and results in an important nutrient pulse to the ground water.