Technical Abstract: This book presents long-term data and cross-site comparisons from 50 sites in the US, including 26 sites in the Long Term Ecological Research Program, 14 in the USFS, and 7 in the USDA Agricultural Research Service. The intent of this book is twofold: (1) to illustrate the importance of long-term data in comparing dynamics across sites and in providing the context for understanding ecological dynamics of relevance to society, and (2) to present long-term ecological data from different sources and a large number of sites in a common format that is easily understood and used by a broad audience. These sites represent six ecosystem types common globally (arctic and alpine [including Antarctica], aridlands, coastal systems, forests, temperate grasslands and shrublands, urban systems), and cover much of the range in average annual temperature and average total annual precipitation for these ecosystems. Variables were selected to characterize either a global change driver (climate, precipitation and stream water chemistry, human demographics) or a biotic response to drivers, primarily by plants and animals. A total of 37 variables were selected for inclusion in this book where: (1) data were available from at least 5 sites for = 10 years, and (2) both the original source data and the associated metadata were available. Key results are: mean air temperature has increased at 24 sites, and annual precipitation has increased at 9 sites with no obvious spatial distribution in either climate variable. Trends in atmospheric chemistry show clear patterns across the continent with reduced deposition of nitrate and sulfate in precipitation through time in the eastern U.S. compared with the West.Patterns in stream water chemistry across sites do not reflect broad-scale patterns in atmospheric chemistry ; thus local conditions have strong influences on chemical inputs to and losses from streams.Human population density has increased at all sites, although at different rates. Biotic data are often idiosyncratic in that they reflect high spatial and temporal variability inherent in biological phenomenon. More variables can be found on the EcoTrends web site (http://www.ecotrends.info).