The chapter explains the philosophy and usefulness of a phased approach in assessing potential environmental hazards of stationary source emissions, and gives examples of the experience and success of the approach to date. Recognition of the problems associated with the environmental release of wastes has resulted in environmental legislation, including the Clean Air and Clean Water Acts and their various amendments, and the Resource Conservation and Recovery Act (RCRA). The U.S. EPA has developed an environmental assessment program for obtaining such information for waste streams. The wastes from industrial and energy conversion processes constitute a major source of exposure for humans and other organisms. Large quantities of such wastes are released annually to the environment in gaseous, liquid, and solid forms. Exposure by organisms is by direct inhalation, ingestion, or absorption through food web transfer.