This chapter presents the methodology, normative data, results from clinical populations and problems associated with the objective measures of sleepiness. The multiple sleep latency test (MSLT) is used in the diagnosis of narcolepsy and the hypersomnias. The maintenance of wakefulness test (MWT) has been used by the FAA and state departments of transportation as a means of screening pilots and commercial drivers for ability to maintain alertness in sedentary work settings. Less research supports the Oxford Sleepiness Resistance (OSLER) and pupillography tests, but the OSLER, which attempts to measure sleep onset without traditional measurement of either performance or EEG, holds promise as a simpler but still time-consuming measure. The tests measure more than a single sleep system and almost certainly reflect the summation of numerous sources of state and trait arousal in addition to the effects of circadian time, prior wakefulness and numerous underlying sleep and arousal pathologies.