Ecotoxicology is the science that seeks to predict the impacts of chemicals 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 prool for application of the science to environmental problem-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 development of methods and expanded recognition of issues for ecotoxicology and for the associated endeavor of environmental management.