An orthodontic appliance 1 for promoting development of a dental arch form in a patient who has an underdeveloped dental arch form is disclosed. The appliance 1 includes an arch-shaped base member 2 that is made of a resiliently flexible material, and a teeth engaging member 5 that encloses at least part of the base member 2. The teeth engaging member 5 defines upper and/or lower dental arch receiving channels 46, 47 and is made of a resiliently flexible material that is softer than the base member and is deformable. The appliance 1 has a resting form in which the resilient materials of the base member 2 and the teeth engaging member 5 are in their resting condition. The appliance 1 can be flexed or deformed out of the resting form to fit the underdeveloped dental arch form into the dental arch receiving channel 46, 47. When deformed the appliance 1 exerts a return force that is directed to returning it to its resting form which in use urges the underdeveloped dental arch to expand into a developed dental arch form.
