Hydropower is based on a simple process, taking advantage of the kinetic energy freed by falling water. In practice, this process is applied in many different ways depending on the electrical services sought and the specific site conditions. Accordingly, there is a wide variety of hydroelectric projects, each providing different types of services and generating environmental and social impacts of different nature and magnitude. This article illustrates the necessity to evaluate each hydroelectric project in relation to the services it provides and to compare electricity supply projects on the basis of equivalent services provided to society. The impoundment and presence of a reservoir stand out as the most significant sources of impacts. However, a reservoir also provides the highest level of electricity supply services: it is the most efficient means of storing large amounts of energy and a hydroelectric plant has the capacity of releasing this energy in quantities that can be adjusted instantly to electricity demand. Furthermore, a reservoir allows for many other uses besides energy storage such as the cost-effective development of run-of-river plants downstream with little environmental impacts.