Understanding the behavior of target fish species is necessary to optimally design, locate, and operate an up- or down-stream fish passage facility. Important fish behaviors are seasonal and daily timing of migration; near field orientation; swimming capability; schooling; response to chemicals; response to other species (competition and predation); and response to physical factors (e.g. illumination, sound, water depth, current velocity, and structure). Recent developments in underwater hydroacoustics, underwater video, and telemetry enable biologists, for the first time, to seriously examine fish behavior in fishways, and in the complex environments at hydroelectric dams. Future advances in fishways will depend on fish behaviorists working with hydraulic engineers to design appropriate fishway environments.