This chapter addresses the prevalence of sleepiness in obstructive sleep apnea syndrome (OSA) and the potential confusion with fatigue, depression and attentional deficits. In clinical practice, sleepiness is in most cases evaluated by the Epworth Sleepiness Scale (ESS). Sleep fragmentation and other sleep structure disturbances are classically considered the main causes of daytime sleepiness in OSA patients. Sleepiness, regardless of its cause, affects driving ability through increased reaction time, inattentiveness or microsleep episodes. Obstructive sleep apnea syndrome (OSAS) and hypertension are linked in a dose-response fashion. This is true even when taking into account confounding factors such as age, alcohol, tobacco consumption, and body mass index (BMI). The beneficial continuous positive airway pressure (CPAP) effect is obtained after only a few weeks of treatment, with quality of life returning to normal. CPAP treatment restores normal alertness except in some patients with residual excessive sleepiness.