Obstructive sleep apnoea (OSA) is characterised by snoring, repetitive sleep-related upper airway obstruction, sleep fragmentation and a range of daytime neuro-behavioural consequences such as impaired vigilance and cognition, increased risk of motor vehicle accidents and reduced work productivity. The global prevalence of OSA is increasing in parallel with rates of obesity. Over the last 2-3 decades a growing body of evidence suggests that OSA may be an independent risk factor for hypertension, increased insulin resistance, type II diabetes, stroke and ischaemic heart disease. Animal models of intermittent hypoxia designed to mimic the nocturnal hypoxia experienced by patients with OSA have shown alterations in vascular endothelial function, raised systemic and pulmonary artery pressure and dysregulation of glucose and lipid metabolism. Epidemiological studies and clinical observational studies suggest that OSA may be an independent risk factor for hypertension, stroke and coronary artery disease, while a number of short-term OSA treatment studies have pointed to improvements in some bio-markers of vascular risk. However, definitive evidence of a causal link between OSA and premature cardiovascular disease and proof that OSA treatment can reduce cardiovascular and metabolic risk will require large-scale randomised controlled OSA treatment trials. Several such trials are currently in progress. If results are positive a new medical treatment will become available for reducing the burden of cardio-metabolic disease.