Monitoring of atmospheric wet and dry deposition §provides insight into trends in pollutant species in §the atmosphere. This study evaluates the spatial and §temporal trends in wet and dry deposition data from §the National Atmospheric Deposition Program/National §Trends Network (NADP/NTN) and the Clean Air Status §and Trends Network (CASTNET). Over the last twenty §years, most of the continental U.S. has experienced §statistically significant trends in one or more ion §species affecting acidic deposition. Some of these §trends appear directly attributable to changes in §regional emissions due to the 1990 Clean Air Act §Amendments. Other trends are influenced by the §equilibrium and reactions between atmospheric §species and other atmospheric processes. Long-term §trends in precipitation chemistry were found to §correlate with trends in gas and particulate §concentrations in the atmosphere for many species.