Trends in biotic structure have been of interest in the United States since the establishment of the Division of Biological Survey in the U.S. Department of Agriculture in the late 1890s. Changes in iotic structure can serve as a bellwether for quantifying the effects of climate change, land-use change, and the spread of exotic species, as well as the loss of rare and endangered species. Considerable evidence suggests that changes in biotic structure can have significant consequences for ecosystem functioning and the provisioning of ecosystems goods and services. In this chapter, we first describe common methods for measuring responses of plants and animals and our rationale for the selection of variables included in this book. We then show graphs of biotic data through time for each site arrayed across the continent.