In accord with the requirements of the Toxic Substances Control Act, new and expanded-use chemicals must be screened for their potential environmental hazards prior to large-scale manufacture and use. This environmental hazard assessment for aquatic systems should have two basic components: aquatic toxicology and environmental chemistry-fate information. Environmental chemistry-fate considers the transport pathways and ultimate disposition of the chemical in the environment, what transformations of the chemical occur, and the rates of these transformations, and it provides an estimate of the expected concentrations of a contaminant in various environmental compartments. A hazard assessment is made by proceeding through a series of testing levels or tiers which develop information on the toxicology of the chemical and its environmental chemistry-fate until a decision can be made that the environmental risk associated with the manufacture and use of the chemical is acceptable or unacceptable. The principles discussed herein are equally applicable to implementing water quality criteria into state water quality standards and assessing the environmental significance of any chemical entering an aquatic system.