This paper provides an overview of the Trung Son hydropower project preparation experience and highlights the innovative features of the project, primarily focusing on the adoption of new approaches in project design and integration of social and environmental concerns. Through disseminating the Trung Son project experience, this paper is intended to contribute to the sustainable scale-up of medium scale hydropower in Vietnam, based on practical experience. This paper was funded by the Australian Agency for International Development (AusAID) as part of a broader activity supporting sustainable hydropower development in Vietnam. Resources from the AusAID-East Asia infrastructure for growth trust fund supplemented those available to the World Bank task team in charge of the project. This paper is made up of three parts: Part A introduces the Trung Son hydropower project. Part B presents highlights from the Trung Son hydropower project, focusing on the main themes addressed during preparation. Part C contains the authors' conclusions about why the Trung Son hydropower project experience matters, and discusses the features that make it a good example for future scale-up of sustainable hydropower development in Vietnam as well as for hydropower development in other countries.