Excessive daytime sleepiness or pathologic sleepiness is a complaint found in patients who experience somnolence at unwanted times and adversely affects their daytime function. Although psychiatric illness, chronic medical illness, or medication side effects may be causes for fatigue, insufficient sleep is the most common cause of excessive daytime sleepiness in the general population. When an individual complains of frank sleepiness, in addition to insufficient sleep, important considerations in these patients are disturbances in the normal homeostatic mechanisms that govern sleep and wakefulness. This article summarizes the clinical presentation, the differential diagnosis, commonly used diagnostic tools, and treatment options for patients complaining of excessive daytime sleepiness.