This chapter discusses obstructive sleep apnea (OSA), which is characterized by repetitive complete or partial upper airway obstruction during sleep, causing intermittent cessation of breathing or reductions in airflow, despite ongoing respiratory effort. Obesity is the major risk factor for development of OSA. OSA is the most common sleep disorder, with a higher prevalence than asthma. During normal sleep, in addition to the changes in muscle tone, there is sleep stage-dependent modulation of cardiovascular physiologic processes, which is severely disrupted in OSA. The most common events contributing to OSA are hypopneas and apneas. Pharyngeal narrowing can occur at multiple sites in patients with OSA, including the retropalatal velopharynx, oropharynx, and retrolingual hypopharynx. Besides daytime somnolence, OSA is associated with depression and neurocognitive defects, possibly as a result of hypoxemia and functional sleep deprivation. Weight loss can lead to significant improvement in OSA. Positional therapy can be used in selected patients with mild OSA.