A number of mechanisms for fish damage during passage through hydro-electric turbines have been postulated, including exposure to rapid pressure changes, to hydraulic shear and turbulence and to cavitation, as well as to collision with the fixed or moving machinery. In a major laboratory programme, experiments were conducted to simulate each of these effects separately so that their relative importance could be determined. Special apparati were developed for this purpose. The main findings are presented and interpreted in relation to fish damage assessment for low - headturbine sites. Particular reference is made to salmonid species.