This review addresses the biophysical mechanisms of image formation in electrosensory systems. These electrical images are used for navigation and object detection by many species of fish, some amphibians, and some mammals. In the active electrosensory systems of fish these images are formed by the fish's own electric organ discharge. In the passive electrosensory systems of fish, amphibians and mammals the images are formed by external electrical sources. In this review we describe the biophysics of image formation, the effects of the organism's passive electrical properties, the role of exploration, and the influence of context on electroreception. We suggest that the basic principles established in these specialized systems be useful for understanding other more common sensory systems.