This chapter summarizes existing knowledge concerning the types, amounts, and sources of hazardous wastes in the nation, selected regions and states. First, hazardous wastes are defined. Then, the sources of hazardous waste generation and how they are keyed to the American economy and the processes of urbanization and industrialization are presented. The chapter shows that one of the reasons that it is so difficult to control hazardous chemical by-products on any but an ad hoc basis is because we do not yet know how much waste has been and is being created. These shortcomings are explained by the long-standing absence of concern with discarded chemical substances, the absence of a clear definition of what constitutes hazardous substances, and wide variations in how the states and federal governments have gone about estimating the size of the waste stream.