The obstructive sleep apnoea syndrome (OSAS) was first identified only 40 years ago and its clinical importance is increasingly recognized. Although now acknowledged as a worldwide problem, which in Western countries affects 2–4% of middle-aged men and 1–2% of middle-aged women, the majority of affected individuals remain undiagnosed. OSAS is strongly associated with obesity but is also increasingly identified in the less obese, in whom a particular craniofacial structure is an important contributory factor. The prevalence of OSAS is likely to be increasing in parallel with the epidemic of obesity currently occurring in many countries. The common presenting complaints are excessive daytime sleepiness and loud snoring. The sleepiness impairs social functioning, work performance and driving ability, and accounts for a large socio-economic burden on the community. Hypertension is an important independent association. The treatment of choice is nocturnal continuous positive airway pressure which is highly effective and is also cost effective.