The complaint of excessive daytime sleepiness, commonly encountered in neurological practice, may arise from a variety of disorders. The list of possibilities spans virtually every major area of medicine, neurology and psychiatry. A clear, detailed history is invaluable in negotiating these numerous diagnostic considerations; however, the symptom may be expressed in terms that do not directly denote somnolence (e.g. 'tiredness' or 'fatigue'). Accurate diagnosis is important, not only because of the negative impacts of sleepiness and its root causes on health and social function, but because excessive sleepiness is generally remediable with appropriate treatment. As our understanding of the neurological underpinnings of alertness and sleepiness deepens, improved treatment methods are bound to emerge.