The objective of Subtask 2 is to identify “Hidden Hydro” in the improvement of performance from existing hydropower facilities based on the case history study and thereby to contribute to further performance improvement through future modernization projects.The Subtask 2 study consists of a review of development methodologies for improving performance from existing hydropower facilities and a case history study of “Hidden Hydro.” Data on case histories were collected basically through literature and questionnaire surveys. Japan served as the subtask coordinator, and the study was conducted with the cooperation of Australia, the European Union, Switzerland and the United States.