This report synthesizes current research relating atmospheric nitrogen (N) deposition to effects on terrestrial and aquatic ecosystems in the United States and to identify empirical critical loads for atmospheric N deposition. The report evaluates the following receptors: freshwater diatoms, mycorrhizal fungi and other soil microbes, lichens, herbaceous plants, shrubs, and trees. The main responses reported fell into two categories: (1) biogeochemical; and (2) individual species, population, and community responses. The range of critical loads for nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is presented. This range spans the range of N deposition observed over most of the country. The empirical critical loads for N tend to increase in the following sequence for four different life forms: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants, shrubs, trees.