Over the past 30 years, the National Atmospheric Deposition Program (NADP) has consistently monitored the nation's precipitation for different chemical constituents and their input to ecosystems around our continent. Our long-term mission is to provide quality-assured data and information in support of research on the exposure of managed and natural ecosystems and cultural resources to acidic compounds, nutrients, base cations, and mercury in precipitation. It has vied to do this in an open and straightforward scientific manner, and to make these data readily available to all interested parties. Since 1977, the NADP has measured approximately 400,000 samples, and has produced hundreds of maps to describe the spatial distribution of concentration and deposition of these chemical constituents. It has summarized the findings annually since 1981. As the network grew, the maps have evolved from simple contour plots to full-color spatially interpreted maps. The underlying consistency of measurements and data quality assurance have made these critically important data available to foster science-based decision-making in many different areas of life, including research, education, agriculture, and policy analysis. Given these measurements and the efforts of many, many people over the years, the NADP has been able to clearly document how the chemical climate of North America is changing. As the maps on the next page show, the concentration of hydrogen ion (or acidity) has decreased markedly since the early 1980s. Concurrently, the deposition of sulfate ion has also decreased significantly, a finding that has been demonstrated over most of the nation, and has been consistent over the history of the network. The NADP's 30 years of effort has been extremely worth-while, based on our results, the demand for the data, and the abundant replication of the summary maps in articles, reports, and presentations. The continued use of the data over these many years is NADP's best return; it is evidence that the data are a valuable and trusted national asset.