Settlement and metamorphosis are induced by environmental cues in many marine invertebrates. These signals emanate from biotic or abiotic material indicating the presence of a suitable habitat. In Cnidarians, pelagic larvae undergo metamorphosis to a sessile polyp. Environmental signals reactivate the morphogenetically inactive larvae and internal mechanisms are turned onto coordinate the development of the polyp. In recent years developmental biologists, ecophysiologists and ecologists have shown a considerable interest in the metamorphosis of Cnidarians. The purpose of this review is to give a survey on what is known about the natural induction of metamorphosis and to provide information on the biochemical and cellular mechanisms underlying the metamorphic events in Cnidarians. The emphasis is on the external and internal signals and their transduction mechanisms used to control metamorphosis.