Electrofishing, a valuable sampling technique in North America for over half a century, involves a very dynamic and complex mix of physics, physiology, and behavior that remains poorly understood. New hypotheses have been advanced regarding "power transfer" to fish and the epileptic nature of their responses to electric fields, but these too need to be more fully explored and validated. Fishery researchers and managers in the Colorado River Basin, and elsewhere, are particularly concerned about the harmful effects of electrofishing on fish, especially endangered species. Although often not externally obvious or fatal, spinal injuries and associated hemorrhages sometimes have been documented in over 50% of fish examined internally. Such injuries can occur anywhere in the electrofishing field at or above the intensity threshold for twitch. These injuries are believed to result from powerful convulsions of body musculature (possibly epileptic seizures) caused mostly by sudden changes in voltage as when electricity is pulsed or switched on or off. Significantly fewer spinal injuries are reported when direct current, low-frequency pulsed direct current (<30 Hz), or specially designed pulse trains are used. Salmoniae are especially susceptible. Endangered cyprinids of the Colorado River Basin are generally much less susceptible, enough so to allow cautious use of less harmful currents for most recovery monitoring and research. However, the endangered catostomid Xyrauchen texanus appears sufficiently susceptible to warrant a continued minimal-use policy. Other harmful effects, such as bleeding at gills or vent and excessive physiological stress, are also of concern. Mortality, usually by asphyxiation, is a common result of excessive exposure to tetanizing intensities near electrodes or poor handling of captured specimens. Reported effects on reproduction are contradictory, but electrofishing over spawning grounds can harm embryos. Electrofishing is often considered the most effective and benign technique for capturing moderate- to large-size fish, but when adverse effects are problematic and cannot be sufficiently reduced, its use should be severely restricted.