Abstract: A magnet structure which is intended for use in retaining dental prostheses and which is arranged when in situ to provide a closed magnetic field. The magnet structure has a high strength permanent magnet element, a carrier for the magnet element and a cap which is fitted to the magnet structure. The carrier is formed entirely from a magnetizable material and it has a pair of spaced-apart pole pieces which sandwich the magnet element, which are connected by a web of metal and which have pole face portions disposed in a common plane with the connecting web. The pole face portions and the connecting web form a substantially flat base surface, and the cap, which is formed of non-magnetizable material, is fitted to the magnet structure in a manner such that it envelopes all but the base surface of the structure.