Obstructive sleep apnoea is the term used to describe the interruption in normal breathing of individuals during sleep. It is caused by collapse of the upper airways during sleep and is strongly associated with obesity. The mainstay of medical treatment is a machine used at night to apply continuous positive airways pressure (CPAP). The machine blows air through the upper air passages via a mask on the mouth or nose to keep the throat open. We searched and reviewed all randomised controlled trials that had been undertaken to evaluate the benefit of CPAP in adult patients with sleep apnoea. Some of the trials had methodological flaws, although more recent studies have begun to use appropriate forms of control. The overall results demonstrate that in people with moderate to severe sleep apnoea CPAP can improve measures of sleepiness, quality of life and associated daytime sleepiness. CPAP leads to lower blood pressure compared with control, although the degree to which this is achieved may depend upon whether people start treatment with raised blood pressures. Oral appliances are also used to treat sleep apnoea but, whilst some people find them more convenient to use than CPAP, they do not appear to be as effective at keeping the airway open at night. Further good quality trials are needed to define who benefits, by how much and at what cost. Further trials are also needed to evaluate the effectiveness of CPAP in comparison to other interventions, particularly those targeted at obesity.