A comprehensive review is made of the observational history of the gegenschein with emphasis on its physical parameters: size, shape, brightness, spectrum, position, and parallax. The theories that have been considered tenable until recently are critically discussed. The libration point, gas tail, and dust tail hypotheses can be disproved, whereas the heliocentric dust cloud hypothesis is consistent with all evidence available. At present the asteroid belt seems to be the most likely source of the material producing the gegenschein. A short discussion of the false zodiacal light is included.