Risk assessment has been suggested as a tool to help manage ecological prob­lems. Ecological risk assessment is usually defined as.the process that evaluates the likelihood that adverse ecological effects are occurring, or may occur, as a result of exposure to one or more stressors. The basic concept, while straightforward, is difficult to apply to any but the simplest ecological problems. Strong reactions, both positive and negative, are often evoked by proposals to use ecological risk assess­ment. Risk assessment applied to relatively simple ecological problems (chemical toxicity being the most common) is popular; there are many vigorous supporters, particularly among scientists, administrators, and politicians. Yet critics are equally vocal. The intellectual history of the risk assessment paradigm as applied to ecolog­ical problems does not follow a neat, linear evolution. A formidable problem in many risk assessments, and especially for complex questions such as addressing the challenge of ecological sustainability, is selecting what ecological component or system is to be considered at risk. This selection is entirely social and political, but estimating the actual risk is technical and scientific. The question of what is at risk must be answered within the political decision-making framework or the results of the risk assessment will be of limited utility. Performing credible risk assessments for complex ecological problems is difficult unless the boundaries of the assessment problem are highly constrained. However, narrowly defining ecological problems produces risk assessments that are of limited relevance in resolving public policy questions.