The objective of this paper is to present a review of the NFP 14+. The studies of this multidisciplinary research program were carried out between 1985 and 1989. They were concentrated on three forest sites at different altitudes, one in the densely populated Mittel-land, one in a protection forest in the Prealps, and one in the Alps proper. All sites are dominated by Norway spruce (Picea abies). The main goal of the case studies was a comprehensive characterization of the sites, giving the opportunity to evaluate the role of air pollution in forest health. The program included various ecological investigations as well as measurements of meteorological parameters, gaseous pollutants and atmospheric deposition. The results indicate neither a temporal nor a spatial dependence of crown defoliation on air pollution for the study sites. Nevertheless, there is experimental evidence that the present-day ozone levels in Switzerland have to be considered as a risk factor for more sensitive tree species.