Significant advances have been made since 1990 in the scientific understanding of the aquatic effects of acid deposition. Models to predict the response of lakes and streams to changing levels of deposition have been extensively tested and improved. There has been an enormous increase in the amount of research being conducted on the effects of atmospheric deposition of nitrogen (N) on aquatic and, especially, terrestrial ecosystems. The experimental approach has shifted heavily into the area of whole-ecosystem experimental manipulations that have been and are being conducted across gradients of atmospheric deposition and other environmental factors throughout northern Europe. Individual investigators are in many cases working at a variety of sites, thus enhancing the comparability of the resulting databases. Results of both the broad-scale and detailed studies have been used to build, test, and validate mathematical models that simulate N processing, nutrient cycling, and water regulation in coniferous forest ecosystems under varying depositional and climatic regimes.