Emissions inventories are the basis for many studies of the relationships between human activities and the environment, especially computer-model studies of the present and future, and are used by the scientific community to link theory to field observations. Within the policy community, they provide some of the most important information needed to formulate international environmental agreements. In North America, Europe, parts of Asia, and a few smaller regions, detailed emissions inventories are available for the principal greenhouse gases, acid-related species, and selected toxics and natural emittants. Internationally-generated, global-scale, gridded emissions inventories have only recently been released for general use. International agreements on emissions reductions are constrained by philosophical, technological, and financial differences among nations, but have nonetheless been shown to be of substantial value in decreasing the emissions of chemical species that clearly cause environmental degradation.