The contamination of freshwater ecosystems by pesticides is described, and the subsequent fates of the pesticides. The acute effects of pesticides on the flora and fauna are examined, and subacute effects, such as those on behaviour, growth, mortality, reproduction, pathology and resistance and genetics, are reviewed. The infinite variety of environmental features of lakes and streams, the multiplicity of chemicals and formulations seen among modern pesticides, and the kinds of effects already noted and measured when pesticides meet plants and animals in freshwater are discussed to emphasize the fact that interactions among these factors can be highly dynamic, productive of manifold effects, and most difficult to predict.