Obstructive sleep apnea syndrome is the most common cause of hypersomnolence in patients referred to sleep disorders centers. This type of sleep apnea is characterized by loud snoring, nocturnal oxyhemoglobin desaturation, and disrupted sleep that leads to daytime hypersomnolence. The anatomic configuration of the pharynx and the physiologic responses to occlusion of the upper airway play a major role in the pathogenesis of this disorder. Polysomnography can accurately identify obstructive sleep apnea, and the multiple sleep latency test allows an objective measurement of daytime alertness. Weight loss and training the patient to sleep in a lateral position are frequently used to alleviate mild cases. Nasally applied continuous positive airway pressure is an extremely effective modality for treating moderate and severe obstructive sleep apnea. Surgical correction of obvious anatomic defects has a role in diminishing obstructive sleep apnea, but the exact role of surgical intervention in patients without obvious anatomic defects remains unknown. The choice of therapy should be tailored to the individual patient with sleep apnea, and careful follow-up is essential to ensure a positive response to therapy.