Diverse animals can detect magnetic fields but little is known about how they do so. Three main hypotheses of magnetic field perception have been proposed. Electrosensitive marine fish might detect the Earth's field through electromagnetic induction, but direct evidence that induction underlies magnetoreception in such fish has not been obtained. Studies in other animals have provided evidence that is consistent with two other mechanisms: biogenic magnetite and chemical reactions that are modulated by weak magnetic fields. Despite recent advances, however, magnetoreceptors have not been identified with certainty in any animal, and the mode of transduction for the magnetic sense remains unknown.