The Conference on the Ecotoxicity of Heavy Metals and Organohalogen Com pounds was held under the auspices of the NATO Science Committee as part of its continuing effort to promote the useful progress of science through international cooperation. Science Committee Conferences are deliberately designed to focus atten tion on unsolved problems, with invited participants providing a variety of complementary expertise. Through intensive group discussion they seek to reach a consensus on assessments and recommendations for future research emphases, which it is hoped will be of value to the larger scientific commu nity. The subjects treated in previous Conferences have been as varied as science itself-e.g., computer software, chemical catalysis, oceanography, and materials and energy research. This volume presents an account of a meeting which evolved from studies within the Science Committee's advisory panel on Eco-Sciences. Environmental monitoring of toxic substances from industrial and agricultural sources is pro ducing a growing volume of data on the quantities of such substances in terres trial and aquatic milieus. Before this information can be used to assess biological effects, knowledge is required of the chemical form of the pollutants, the mecha nisms by which they enter and move through organisms, their concomitant transformations, the nature of the toxic reactions within tissues, and the way in which the physiology and behavior of individuals is affected.