A mandibular advancement device (10, figure 3) has an upper retainer (14) positionable in front of teeth of a user's upper jaw and connected to a lower retainer 16 positionable behind teeth of a user's lower jaw. The lower retainer has a support structure 56 at its rear. The support structure has a shape and/or is made from a material such that, in use, a relatively high force is applied to two or more rearward teeth of the lower jaw to move the lower jaw forwards, and a relief region 54 towards the front of the lower retainer applies a relatively low or no force on one or more forward teeth of the lower jaw. Also claimed is a mandibular advancement device which applies force closer to the gumline than to the biting surfaces of the teeth.
