Emphasis is placed on studies conducted in regions that contain large numbers of acid-sensitive aquatic systems. Regions in which aquatic resources are either not very sensitive or are primarily influenced by environmental perturbations other than acidic deposition receive less coverage. Few data are available from high-elevation sites where many of the most sensitive aquatic and terrestrial resources are located. In addition, knowledge is limited of the amounts of deposition other than wet deposition. Virtually all of National Acid Precipitation Assessment Program’s major aquatic modeling and integration efforts leading up to the Integrated Assessment focused predominantly on the potential effects of S deposition. A smaller N research effort has been directed at investigating effects of N deposition on aquatic ecosystems. The discrepancy between Gran titration Acid neutralizing capacity (ANC) and calculated ANC caused by organic acid influence and/or differences in defining the proton references for Al have major implications for aquatic effects assessment activities.