This chapter provides a summary of key technical references useful in environmental impact studies on large dams and hydroelectric projects. It reviews the numerous types and classifications of aquatic impact models a complete. Identification of potential impacts should be an early activity in an environmental impact study. Environmental impact studies require consideration of the baseline conditions of the environment. The most important technical activity in an environmental impact study for a dam/reservoir project is the scientific prediction of the effects of project construction and operation. Impact prediction can be achieved via a qualitative approach which relies on general knowledge of the impacts of similar projects, or specific results of comprehensive studies of similar projects. It also includes a quantitative approach based on the use of simple mass balance and environmental dilution calculations and a quantitative approach based on the use of mathematical models for multiple environmental factors.