The job of the responsible zoologist should be to assess or attempt to predict the consequences of any effluent or other environmental disturbance as objectively as possible, bearing in mind both the needs of conservation and the reasonable demands of man. This book has been written to assist this process of assessment and prediction as far as thermal discharges are concerned. It gives a summary and review of the biological studies which have been carried out in various habitats and in response to a variety of problems related to cooling water usage, particularly on the large scale as in thermal power stations. By discussing and summarizing the hugely variable field studies, the book should help the reader to unravel the complexity of the effects of at least one kind of effluent. It should also be of use in the future planning of industries which use cooling water and for the controlling authorities whose task is to set realistic constraints on these industries.