Sleep apnoea is the popular term for obstructive sleep apnoea-hypopnoea syndrome (OSAHS). Apnoea may be "central", in which there is cessation of inspiratory effort, or "obstructive", in which inspiratory efforts continue but are ineffective because of upper airway obstruction. It affects up to 24% of men and 9% of women in the US, with obesity being a major determinant.We conducted a systematic review and aimed to answer the following clinical questions: What are the effects of treatment for severe obstructive sleep apnoea-hypopnoea syndrome? What are the effects of treatment for non-severe obstructive sleep apnoea-hypopnoea syndrome? We searched: Medline, Embase, The Cochrane Library and other important databases up to May 2006 (Clinical Evidence reviews are updated periodically, please check our website for the most up-to-date version of this review). We included harms alerts from relevant organisations such as the US Food and Drug Administration (FDA) and the UK Medicines and Healthcare products Regulatory Agency (MHRA).We found 25 systematic reviews, RCTs, or observational studies that met our inclusion criteria. We performed a GRADE evaluation of the quality of evidence for interventions.In this systematic review we present information relating to the effectiveness and safety of the following interventions: measures aimed at improving compliance, nasal continuous positive airway pressure, oral appliances, and weight loss.