In summary, alveolar hypoventilation can be associated with a diverse group of disorders, collectively referred to as the hypoventilation syndromes. Most have associated hypercapnia and hypoxemia while awake, with a significant worsening in gas exchange during sleep. In some disorders, gas exchange abnormalities are manifested only during periods of sleep. Signs and symptoms suggestive of the underlying disorder leads one to investigate for associated hypoventilation. Proper diagnosis allows the implementation of appropriate therapy, which may both improve gas exchange and associated symptoms, and impact overall survival.