ecause the desire to sleep is an everyday occurrence, the possible B negative consequences of excessive sleepiness are easy to picture.Everyone has experienced how compelling strong sleepiness feels. It can have the potency of a powerful drug and can undermine vigilance, perception, tlunking, judgment, and emotional control. Excessive sleepiness is suspected to have been a factor in the Chemobyl nuclear catastrophe (Dinges, Graeber, Carskadon, Czeisler, & Dement, 1989;Mitler et al., 1988). The accurate measurement of excessive daytime sleepiness serves the attainment of valid diagnosis and treatment planning. A fundamental goal is to prevent the unfortunate outcomes that can occur because of undocumented and untreated disorders of excessive sleepiness.The present chapter is about the goal of accurate sleepiness measurement. The clinical benefits of formal sleepiness testing are explained. The inconsistent relationship between the subjective and physiological aspects of sleepiness is lughlighted because of its importance to clinical decision making. There is significant risk for inaccurate clinical judgments when depending solely on patient self-descriptions of sleepiness. Current clinical and research measures of daytime sleepiness and standard procedures for directly recording the physiological tendency to fall asleep are reviewed in detail. The future is touched on in a closing discussion about the assessment of treatment outcome. Some of the dormation presented suggests the future is rich with potential research opportunities for creating and refining approaches with sleepiness measurement. 209