An evaluation of the consequences of environmental contamination requires an understanding of the extent to which it is responsible for changes in populations of organisms in the affected area. Population change is not solely related to mortality which may be observed, but depends also on the population dynamics, stock size and survival strategy of the species affected. Population changes affecting species of commercial or sentimental importance or whose diminution is followed by major community adjustment, are regarded more seriously than those of other species. Community adjustment to stress by contaminants may be subtle, difficult to detect and still more so to evaluate. In some instances it is possible to unravel the causative agents, but studies of community response to stress have lagged behind those at lower organizational levels of the individual, tissue or cell. The succeeding papers address questions arising from these considerations.