The term "functional appliance" refers to a variety of removable appliances designed to alter the arrangement of various muscle groups that influence the function and position of the mandible in order to trasmit forces to the dentition and the basal bone. Typically these muscular forces are generated by altering the mandibular position sagittally and vertically, resulting in orthodontic and orthopedic changes. We can say functional appliances are removable or fixed orthodontic appliances that are held in contact with both upper and lower dental arches. The appliances hold the mandible in a postured position away from the normal rest position.