Abstract Ecotoxicology provides a basis for making decisions on the likely impact of a chemical or effluent on the aquatic environment. It encompasses laboratory ecotoxicity tests of various types to explore relationships between exposure and effect under controlled laboratory conditions, through to studies of the effects of chemicals or effluents under a variety of ecological conditions in complex field ecosystems. This paper will focus on the value of laboratory ecotoxicity tests as a tool in assessing water quality. Laboratory tests are valuable (i) in deriving and assessing water quality criteria, (ii) for screening and ranking chemicals and predicting their hazard and risk, (iii) for establishing dilution levels of chemicals or effluents prior to discharge into water bodies, (iv) in determining cause-effect relationships in post-impact studies, and (v) for establishing and validating field bioindicators. Both the advantages and deficiencies of using ecotoxicological testing for these purposes are illustrated from research with pesticides, metals and sediments. Use of a combination of both laboratory- and field-based ecotoxicology studies is important to gain a full understanding of the effects of chemicals at the ecosystem level.