The Chesapeake Bay Basin Comprehensive List of Toxic Substances is a compilation of the toxic substances detected in, released, or applied to all media (water, soil, sediment, tissue and air) within the Chesapeake Bay basin. A substance's inclusion in this list does not constitute evidence of potential or existing environmental impact. It merely documents a finding or measurement in some media at some point in time. This Comprehensive List serves two specific purposes: It is the central listing of toxic substances around which the Chesapeake Bay Program's Toxics Data Base has been structured and organized, and it defines the 'universe' of toxic substances from which future revisions and updates to the Chesapeake Bay Toxics of Concern List will be ranked and identified. The Comprehensive List is being published as a document to meet other potential uses by the Chesapeake Bay Program agencies such as defining parameter lists for toxics monitoring programs.