Oral appliances are increasingly recommended for selected patients with obstructive sleep apnoea (OSA) and those who do not tolerate nor prefer continuous positive airway pressure. The most commonly used oral appliance advances the lower jaw during sleep, the so-called mandibular advancement device (MAD). Patients seek treatment because of disturbing snoring, daytime symptoms, apnoeas that disturb sleep and the longer term consequences with regard to cardiovascular risks. MADs reduce the apnoea-hypopnoea index, although to various degrees among patients. Effects on daytime sleepiness have been observed mainly among the more severe OSA patients. Blood pressure may be reduced in MAD-treated OSA patients. There is, however, uncertainty about which patients will respond to this therapy in terms of apnoea reductions, decreased sleepiness and other symptoms, and reduced risk for future impaired health. The occurrence of side-effects also remains difficult to predict at present. The majority of sleep apnoea patients suffer from various comorbidities in terms of cardiovascular diseases, type 2 diabetes and depression. The most recent findings indicate that phenotyping of patients, considering various aspects of this multifaceted disease, will shed more light on the indications for MADs in patients with nightly sleep breathing disturbances. This review summarises the most recent knowledge about MAD treatment.