The topics related to the problems of fish habitat conservation in rivers and streams are treated as a part of Eco-Hydraulics. Three key factors: (1) essential conditions needed for streams where fish can live, (2) essential factors to complete migration, and (3) important characteristics of migrating fish are summarized. Then, fundamental concepts about fish passage are described, especially considering the latter two key factors. In the second section on instream flow, flow rate problems for fish and a recent tool to analyze the problem are discussed. The second section focuses on the first key factor, by introducing the PHABSIM portion of IFIM.