Obstructive sleep apnea (OSA) is a common form of sleep-disordered breathing that occurs due to recurrent collapse of the upper airway with inspiration. Large epidemiologic studies have established that OSA is a risk factor for developing hypertension. The pathophysiologic mechanism of this relationship is due to the distinctive pattern of intermittent hypoxia seen in OSA. This pattern increases sympathetic tone, oxidative stress, inflammation and endothelial dysfunction. These processes can all lead to persistent elevation of blood pressure beyond the obstructive events. OSA should be considered as part of the workup of patients with hypertension. Treatment of OSA with continuous positive airway pressure has an effect on hypertension control and risk reduction of cardiovascular diseases. This review discusses the pathophysiology and causal relationship between OSA and hypertension, along with the cardiovascular effects of treatment of OSA.