The paper presents two research approaches that address problems encountered in evaluating the effects of complex mixtures of chemicals on aquatic systems. The concept of ambient toxicity testing is applied to the impact of effluents in freshwater receiving waters (the concept also applies to saltwater systems), where measurement of toxicity is made without attempting to identify the toxics. Another approach develops structural and functional indices that can be used to evaluate impact of chemicals on communities maintained under controlled conditions in the laboratory. One approach is concerned with chemicals already in the environment; the other, with developing ecosystem level indices used to evaluate chemicals before they reach the environment. Also, applicability of laboratory-derived data to field situations is discussed.