Over the past 30 years OSA has evolved from being a very rare disorder, almost solely characterised by loud snoring and daytime sleepiness, to a common systemic disorder with multiple adverse consequences, which affect a substantial percentage of the middle-aged population. Obesity is the strongest risk factor, but not the only pathogenic mechanism. Other important mechanisms include differences in craniofacial dimensions and even lifestyle factors such as smoking, alcohol consumption and sleeping position. Given the systemic effects, there is developing evidence for the role of OSA as an independent risk factor for: excessive sleepiness with increased risk of traffic accidents, hypertension, cardiovascular disease (including atherosclerosis), myocardial infarction, stroke, impaired glucose metabolism, neurodegeneration, and both cancer mortality and incident cancer. Subjects with untreated OSA have increased mortality and are at higher risk when undergoing surgical procedures. The major clinical challenge today is how simplified diagnostic procedures can confirm or reject the possibility of OSA in various situations.