The National Atmospheric Deposition Program (NADP) is a cooperative program that provides long-term, high-quality precipitation and air chemistry data along with supporting measurements to evaluate temporal and spatial trends in atmospheric deposition.  The NADP monitoring networks measure acids, nutrients, and base cations in precipitation and atmospheric concentrations of ammonia.  The Clean Air Status and Trends Network (CASTNET) and Interagency Monitoring of Protected Visual Environments (IMPROVE) offer long-term trend measurement of particulate matter (PM) nitrate and sulfate species amongst other data results.  A number of NADP sites are either co-located or within 50 Km of CASTNET or IMPROVE samplers, allowing the opportunity to compare long-term measurement results.  These networks make their data freely available for download and use, supporting a broad spectrum of research and regulatory initiatives, including multi-pollutant source/receptor relationships, deposition impacts on terrestrial and aquatic ecosystems, and biogeochemical cycling of pollutants.  Implementation of the Clean Air Act has effectively reduced emissions of NOx and SO₂ in the United States and the impact of these policies can clearly be observed in the spatial and temporal trends documented by the NADP’s National Trends Network (NTN) and CASTNET and IMPROVE datasets.  The NTNs 40 year history has shown reductions of up to 50% and 70% of nitrate and sulfate wet deposition, respectively, at individual locations in the Midwest.  However, in contrast to these reductions of acid anions, the NADP networks have documented increased atmospheric concentrations of ammonia and deposition of ammonium in specific regions of the United States.  The Midwest, which experiences significantly higher ammonium deposition than the rest of the nation, has shown little change in nitrogen deposition in recent years. In contrast, areas with historically lower ammonium deposition, such as the Western states, have recently exhibited notable increases in ammonium deposition.  The presentation will highlight major observations in nitrogen species monitoring, including long term trends in nitrate and ammonia. Presented results will highlight comparisons of NTN, CASTNET, and IMPROVE results and the importance of national networks to document improvements and emerging issues of pollution concentrations.