Abstract: A mandibular advancement device includes a lower part, a lower flange, an upper part and an upper flange. The lower part is releasably attachable to at least a portion of a lower jaw. The lower flange has a lower engagement surface extending upwardly from the lower part. The upper part is releasably attachable to at least a portion of an upper jaw. The upper flange has an upper engagement surface extending downwardly from the upper part. The device includes a lower one way ratchet means configured to only allow uni-directional posterior movement of the lower flange relative to the lower part and/or an upper one way ratchet means configured to only allow uni-directional anterior movement of the upper flange relative to the upper part. When the device is fitted to a patient's jaws, the upper and lower engagement surfaces engage to cause anterior advancement of the lower jaw.