About 30% of individuals in the Swedish population will at some stage during life have treatment with orthodontic appliances. In more severe cases, when orthodontic treatment is not considered sufficient enough to correct the malocclusion, the orthodontic treatment is combined with orthognathic surgery. For these cases, a satisfying jaw relation is achieved by surgically moving the maxilla and/or the mandible into a pre-planned position. Patients due to be treated with orthognathic surgery often suffer from an impaired masticatory function, symptoms from the masticatory muscles or temporomandibular joints (temporomandibular disorders), headaches as well as dissatisfaction with their facial aesthetics. Since orthognathic treatment is expensive, in many cases arduous to the patient and not without complications, it is important to assess the treatment outcome and if this is satisfying for the patients. Previous studies that have examined the outcome after orthognathic treatment have had diverging study designs and have come to different conclusions with regard to both temporomandibular disorders and masticatory function. The overall aim of this thesis was to assess and compare the frequencies of temporomandibular disorders and the masticatory function in patients with dentofacial deformities before and after orthognathic treatment. THE THESIS IS BASED ON THE FOLLOWING STUDIES: Paper I is a systematic literature review aiming to, in an evidence-based approach, answer the question whether orthognathic treatment affects the prevalence of signs and symptoms of temporomandibular disorders. The review encompasses the period from January 1966 to April 2006 and was further extended to May 2013 in the frame story of this thesis. CONCLUSIONS IN PAPER I AND THE COMPLEMENTARY SURVEY: There is insufficient scientific evidence for a decrease of sub diagnoses of temporomandibular disorders after orthognathic treatment. There is limited scientific evidence for a reduction of masticatory muscle pain on palpation after orthognathic treatment. There is insufficient scientific evidence for an effect on temporomandibular joint pain on palpation and temporomandibular joint sounds from orthognathic surgery. Further controlled, well-designed studies assessing temporomandibular disorders before and after orthognathic treatment are needed to consolidate strong evidence considering treatment outcomes. Papers II and III are studies comparing frequencies of temporomandibular disorders in patients with dentofacial deformities with a control group. The patients were referred for a combined orthodontic and orthognathic treatment to correct their malocclusion. The control group comprised individuals with normal occlusion or minor malocclusion traits not in need of orthodontic treatment. In Paper III, temporomandibular disorders were longitudinally analysed by assessing and comparing frequencies before and after orthognathic treatment. All individuals in the studies were diagnosed according to the research diagnostic criteria for temporomandibular disorders. CONCLUSIONS IN PAPERS II AND III: Patients due to be treated with orthognathic surgery had more signs and symptoms of temporomandibular disorders and a higher frequency of diagnosed temporomandibular disorders compared with the age- and gender matched control group. Patients with dentofacial deformities, corrected by orthodontic treatment in conjunction with orthognathic surgery, had a positive treatment outcome in respect of myofascial pain and arthralgia. After treatment the frequency of temporomandibular disorders . in the treatment group was low and at an equivalent level of that in the control group. Paper IV evaluates the self-estimated masticatory ability and the masticatory performance before and after orthognathic treatment in the same individuals as in Paper II and III. CONCLUSIONS IN PAPER IV: Masticatory ability and performance increased after orthognathic treatment. The number of occlusal contacts and severity of overall symptoms of TMD influenced both the masticatory ability and performance. Open bite had a negative effect on masticatory performance.