The evaluation of a patient with excessive sleepiness requires that a detailed sleep history is taken with medical, psychiatric, and psychosocial factors considered, and a differential diagnosis developed. This chapter deals with the important elements in the clinical evaluation of patients with excessive sleepiness. The social history should be elicited, particularly relationships with other family members, and including determination of any financial, personal or social stresses that may contribute to sleep disturbance. Some sleep disorders including sleep apnea syndrome, narcolepsy, recurrent hypersomnia as well as restless legs syndrome, have a familial tendency. The physical examination ideally should be comprehensive and focus on respiratory, cardiovascular, gastrointestinal, endocrine and neurological evaluation. The patient who is sleepy may be asked to complete various tests of performance, such as a psychomotor vigilance test (PVT), or other tests of cognitive ability.