This overview summarizes current knowledge of effects, side effects and indications for oral appliances in the treatment of patients with obstructive sleep apnea (OSA) and snoring. Mandibular repositioning appliances (MRAs) in various designs represent the most evaluated type of oral appliance. MRAs relocate the lower jaw forward and downward in order to prevent sleep-induced pharyngeal collapse. Short-term success rates with MRAs of over 80% of selected OSA patients have been reported, but figures vary. MRAs are indicated for patients with mild to moderate OSA, men with supine-dependent OSA and women. Patients with initial treatment success may benefit from MRAs over a long time, provided that they do not gain weight and wear high quality devices. There are few randomized controlled trials of treatment effects from MRAs on OSA, particularly regarding symptoms. MRA design, methodology and the teamwork between dentists and physicians has an influence on the treatment outcome. Continuous positive airway presssure is the primary treatment for patients with more severe OSA because of its superior effects compared with MRAs. Another drawback of MRAs is their dependence on healthy teeth and the risk of dental side effects. It is concluded that MRAs are indicated for selected patients with OSA, but more research is needed.