This special issue of Energy Policy is dedicated entirely to hydropower and this introductory article provides an overview of the contents. It is divided into three main subject areas: the present situation and history, the main policy issues, and the future of the hydropower industry. The first area covers the evolving context, the diversity, and the potential of the hydropower industry, and also reports on hydropower in China, the country with the largest remaining potential. The policy area covers the social and environmental effects, the regulatory framework and public acceptance issues. The discussion of the future describes the changes that are underway and still need to be made, to ensure that hydropower will continue to serve the needs of society for electricity and several other important services. Hydropower projects can yield large benefits to society, but they also entail costs, and well-informed decision-making processes are necessary to ensure the best outcomes for a region or a country. The authors who contributed to this special issue hope that their articles will provide some of the information needed for good decision making.