Little has been done, until recently, to develop a methodology to analyze and estimate quantitative risk due to natural and anthropogenic ecological phenomena except for the effects on humans. Thus, the current efforts outside human health endpoints are not prejudiced by previous work. Following the EPA Science Advisory Board, we will here identify the two areas as human health and ecological health for convenience, although this is an artificial separation. In the area of human quantitative risk many estimates have been made, particularly for the cancer endpoints. However, to date, no adequate theoretical or philosophical basis has been developed in the area of ecological risk assessment. Ecological quantitative risk assessment is being developed in a systematic way, starting with a careful examination of the questions being asked and those that should be asked. Once the concepts are developed, ecological quantitative risk assessment should develop rapidly. The development of ecological quantitative risk assessment is still in its early stages, and many gaps and potential gaps for data and information are being found and some are discussed in the next chapters.