This article is a review of the current evidence that links systemic hypertension with obstructive sleep apnoea. Whilst a causal association has been suspected for some time, the day to day variability of both blood pressure and sleep apnoea severity, and clustering of confounding cardiovascular risk factors in sleep apnoea patients has made this association difficult to prove. There is unassailable evidence that obstructive apnoeas raise blood pressure acutely in both animal models and humans, through a combination of autonomic and state dependent arousal with some mechanical influences, and these rises can be controlled by nasal continuous positive airway pressure. Thus, although repetitive apnoeas alter the blood pressure variability and raise sleeping blood pressure in patients with OSA and sophisticated animal models have demonstrated increases in daytime blood pressure after the onset of OSA in the short term, such effects on diurnal BP have yet to be proven in humans. Recent rigorously designed large epidemiological studies have proven an independent association between OSA and systemic hypertension in both general and sleep clinic populations, with closely matched case control series also reporting raised blood pressure in OSA patients. A direct temporal causal association between the onset of obstructive sleep apnoea and raised blood pressure is expected to be confirmed by longitudinal data from the continuing epidemiological population studies. Finally, several studies on the beneficial effects of nasal continuous positive airway pressure in reducing blood pressure in OSA patients have preliminary results in abstract form, with one published in full.