Why is the construction of hydropower dams, which generate energy that is renewable and cleaner than oil, coal, or nuclear power, increasingly attacked in the press and assaulted by many critics? The answer, a good part of it, is that the environmental critique of dams has shifted considerably from their physical to their social impacts. During the last decade the thrust of this social critique has been sharpened, new arguments are being used, while practical responses to the newly raised social issues are lagging. The adverse social impacts of hydropower dams, if unaddressed, can be very serious. Unless these negative social impacts are mitigated by governments through explicit policy, legislation, and financial resources, and by planners through socially sensitive resettlement planning, hydropower expansion in developing countries will be retarded by social tensions and by increasing political and environmental opposition.