Excessive daytime sleepiness and fatigue are common complaints in the sleep clinic. The objective evaluation and quantification of these symptoms is important for both the diagnosis of underlying health problems and for gauging treatment response. The multiple sleep latency test measures physiologic sleepiness, whereas the maintenance of wakefulness test (MWT) aims to measure manifest sleepiness. Neither test correlates well with subjective measures of sleep such as the Epworth sleepiness scale and the Stanford sleepiness scale. Although in the past methodological testing differences existed, in 2005 updated practice parameters were published, promoting the standardization of testing procedures. In recent years, there has been an effort to document daytime sleepiness when associated with occupational risk. However, these laboratory-based tests may not reflect or predict real-life experience. Normative data for both tests, particularly the MWT, are limited, and are inadequate for the evaluation of pediatric patients, shift workers, and others.