Hydropower industry practitioners design and implement a large number of environmental and social mitigation measures within the context of hydroelectric projects. However, the effectiveness of such measures is relatively unknown. Indeed, very little has been published on the results of such measures, especially when the results obtained have proven to be inconclusive. To address this concern, the authors carried out a review of current mitigation practices. The detailed results of the review can be found in the following International Energy Agency (IEA) publication: Hydropower and the Environment, Annex III-Subtask 6, April 2000. The information gathered within the scope of the study was based upon: (a) a review of 147 recent scientific publications; and (b) a survey of nine international hydropower producers using a detailed questionnaire. This paper summarises the results of the study. These are based upon actual case studies and upon the experience of hydropower industry practitioners and of professionals from the environmental and social sciences.