Obstructive sleep apnea is more prevalent in patients with hypertension than in the general population and many with obstructive sleep apnea also have hypertension. Obstructive sleep apnea increases the risk of hypertension-related morbidities such as stroke, heart failure, and premature death. Are such associations coincidental or causal and if the latter, what are their implications for clinical practice? Despite compelling epidemiological and mechanistic links between obstructive sleep apnea and hypertension, the effect in clinical trials of the treatment of obstructive sleep apnea on blood pressure has been modest and variable. The purpose of this review is to summarize our present understanding of: (1) the relevant epidemiology and mechanisms that might be responsible for the bidirectional relationship between obstructive sleep apnea and hypertension; and (2) available evidence regarding the effect of treating obstructive sleep apnea on blood pressure.