Sleepiness is a common, but not necessary symptom of the obstructive sleep apnea syndrome (OSA) and is a frequent chief complaint of patients with OSA who seek medical attention. While sleepiness may seem simple in nature, the underlying mechanisms producing daytime sleepiness in OSA are complex and poorly characterized. Moreover, the meaningful assessment of pathological sleepiness is frequently far from straightforward. Effective treatment of OSA is generally expected to resolve or ameliorate daytime sleepiness. An unknown percentage of treated OSA patients, however, remain sleepy during waking hours. The assessment and treatment of residual sleepiness in treated OSA can range from simple to difficult, depending on the nature and causes of the continued sleepiness. Recently, however, data from clinical trials have been generated which provide direction in the evaluation and management of the OSA patient suffering residual daytime sleepiness.