This chapter presents and discusses the state-of-the-art concerning the basic terminology and legal definitions related to waste, in particular solid waste, in the United States and the European Union (EU). The activities of human society are always accompanied by waste generation. The fundamental environmental issue in industrial and developing countries throughout the world is the way to best identify and manage waste streams. The term, “hazardous waste,” means a solid waste, or a combination of solid wastes, which because of its quantity, concentration, or physical, chemical, or infectious characteristics may (1) cause, or significantly contribute to an increase in mortality or an increase in serious irreversible, or incapacitating reversible, illness, or (2) pose a substantial current or potential hazard to human health or the environment when improperly treated, stored, transported, disposed of, or managed. Waste terminology and definitions used in the EU directives, catalogues, and standards are intended to be the base of common language assuring compliance with regulatory provisions or a contractual situation among parties in waste management chain. The waste legislation in the EU, alike national waste legislation of the majority of member states, primarily addresses environmental and public health, and to a lesser extent, resource concerns.