Ecotoxicology is the science that seeks to predict the impacts of chemi cals upon ecosystems. This involves describing and predicting ecological changes ensuing from a variety of human activities that involve release of xenobiotic and other chemicals to the environment. A fundamental principle of ecotoxicology is embodied in the notion of change. Ecosystems themselves are constantly changing due to natural processes, and it is a challenge to distinguish the effects of anthropogenic activities against this background of fluctuations in the natural world. With the frustratingly large, diverse, and ever-emerging sphere of envi ronmental problems that ecotoxicology must address, the approaches to individual problems also must vary. In part, as a consequence, there is no established protocol for application of the science to environmental prob lem-solving. The conceptual and methodological bases for ecotoxicology are, how ever, in their infancy, and thus still growing with new experiences. In deed, the only robust generalization for research on different ecosystems and different chemical stresses seems to be a recognition of the necessity of an ecosystem perspective as focus for assessment. This ecosystem basis for ecotoxicology was the major theme of a previous pUblication by the Ecosystems Research Center at Cornell University, a special issue of Environmental Management (Levin et al. 1984). With that effort, we also recognized an additional necessity: there should be a continued develop ment of methods and expanded recognition of issues for ecotoxicology and for the associated endeavor of environmental management."