Excessive daytime sleepiness (EDS) or somnolence is common in our patients and in society in general. The most common cause of EDS is "voluntary" sleep restriction. Other common causes include sleep-fragmenting disorders such as the obstructive sleep apnea syndrome. Somewhat less familiar to the clinician are EDS conditions arising from central nervous system dysfunction. Of these so-called primary disorders of somnolence, narcolepsy is the most well known and extensively studied, yet often misunderstood and misdiagnosed. Idiopathic hypersomnia, the recurrent hypersomnias, and EDS associated with nervous system disorders also must be well-understood to provide appropriate evaluation and management of the patient with EDS. This review summarizes the distinguishing features of these clinical syndromes of primary EDS. A brief overview of the pharmacological management of primary EDS is included. Finally, in view of the tremendous advances that have occurred in the past few years in our understanding of the pathophysiology of canine and human narcolepsy, we also highlight these discoveries.