Downstream passage through hydroelectric facilities is a significant area of concern regarding the protection of emigrating silver-phase eels (Anguilla spp.) because eels residing upstream of dams must emigrate to achieve their biotic potential and because their complex life history, relatively large size, and unique physiology may result in high vulnerability to injury or mortality during passage, in tum resulting in lost productivity on the species/population level Efforts to develop and implement upstream passage of juvenile American eels over the past decade or so presumably will result in significant increases in numbers of adult eels requiring downstream passage as they reach maturity. Hydroelectric projects are often hydraulically complex and they present multiple passage routes, including turbines, spill and/or deep water gates, and bypasses that pose different risks for downstream passage, as a result route selection can have significant impacts on reproductive potential. Understanding the combination of passage route selection and relative survival by passage route is necessary to evaluate the impacts of a particular project, and subsequently estimate cumulative impacts in a basin where multiple dams are present. The route of passage used by emigrating eels is dependent on physiology and behavior as well as physical processes and project operations. Biotelemetry techniques, such as PIT, radio telemetry, and acoustic tags are suited to examine the behavior of emigrating eels at hydroelectric projects and have been used at several sites. Results of biotelemetry and passage survival studies have been applied to develop short-term and permanent downstream protection measures and can assist in the design of hydro-mechanical passage measures. Additionally, useful information regarding migratory behavior and passage is gained, sometimes challenging our pre-existing understanding of best practices.