The dams reviewed in this paper w Three Gorges in China, Ilisu in Turkey and Urra in Colombia -- are controversial and the assessment of their social impacts represents a challenge. This paper emphasizes the complexity of the institutional setting and social impacts of these projects as well as the specific problems raised by their assessment, which result from the magnitude, intensity and visibility of these impacts. The paper draws lessons from these projects on SIA methods, impact perception, the analysis of project alternatives, the design of mitigation and compensation measures, social monitoring and follow-up, as well as ethical boundaries. [ABSTRACT FROM AUTHOR]