Hydroelectric development projects, which frequently encounter ethical dilemmas, cannot be subject only to legal rules. Ethics are also at stake, especially in controversial situations. The purpose of this paper, which is based on an IEA report, is to find ethical principles that may help in the search for standards of conduct regarding hydropower development and the environment. The principles serve as the criteria for an article on legal and regulatory frameworks that also appears in this issue. This paper begins by restating, according to the IEA report, recurrent dilemmas raised by hydroelectricity projects (the protection of nature versus the satisfaction of essential human needs, distribution of wealth, the rights of affected communities, and the diversity of rules and cultural differences). The paper then presents and describes the five ethical principles discussed in the IEA report (stewardship; participatory decision-making; prudence and control; fairness and justice; and optimality). These principles are derived from international law and can guide decision-makers faced with dilemmas arising from hydropower projects. Accordingly, this text is a complement to the article on legal and regulatory frameworks that also appears in this issue.