Sharks and rays have a long evolutionary history as major predators in marine ecosystems, but the biological functions and selective pressures that shape the evolution of their ampullary electrosensory system are poorly known. The ampulla of Lorenzini is the functional electrosensory unit that consists of a small subdermal ampulla and a canal that projects to a surface pore on the head or pectoral fins. The sensory epithelium of the ampulla wall detects differences between the potential at the skin pore and internal potential of the animal, and stimulates neural transmission of information about the physical features of an external field to the brain. Natural weak electric stimuli include polar fields from bioelectric sources and induced fields from physical sources in the environment. Neurophysiological studies show that the ampullary electrosense responds to electric field gradients as low as 20 nV/cm, and behav- ioral studies show responses to gradients of 1-5 nV/cm. Elasmobranch fishes show behavioral responses to bioelectric stimuli produced by natural prey, mates, consexuals and potential predators. Numerous models exist for electrosensory navigation, but they remain to be rigorously tested. Recent work shows age-dependent changes in the response properties of the electrosense among embryo, juvenile and adult stages and are proposed to reflect ontogenetic adaptations to their changing environments. In addition, the electrosense response properties are seasonally modified by the periodic expression of gonadal steroids and may serve important modulation of sensory function during reproductive behaviors. Future work should continue to investigate different biological contexts in which the electrosense is used by elasmobranch fishes, and to test the selective forces that may have shaped the evolution of this remark- able sensory system.