Until recently, procedures used to derive water quality criteria for aquatic life were not well defined and few principles were identified. On 28 November 1980 the United States Environmental Protection Agency published “Guidelines for Deriving Water Quality Criteria for the Protection of Aquatic Life and Its Uses” in the Federal Register. These have been subsequently revised and renamed “Guidelines for Deriving Numerical National Water Quality Criteria for the Protection of Aquatic Life and Its Uses” and are referred to as the “National Guidelines.” In addition, guidelines have been developed for deriving site-specific criteria either by modifying national criteria or by using other appropriate information. Procedures for deriving water quality criteria and for assessing hazard to aquatic life have many similarities because both make use of information from many areas of aquatic toxicology. In addition, both assume that the science has developed sufficiently that these activities are feasible and desirable and that appropriate rationales are available for making necessary decisions concerning various technical issues.