This paper is a review of the literature on the use of polysomnography in the diagnosis of sleep disorders in the adult. It is based on a search of MEDLINE from January 1966 through April 1996. It has been reviewed and approved by the Board of Directors of the American Sleep Disorders Association and provides the background for the accompanying ASDA Standards of Practice Committee's Parameters for the Practice of Sleep Medicine in North America. The diagnostic categories reviewed are: sleep-related breathing disorders; other respiratory disorders; narcolepsy; parasomnias and sleep-related epilepsy; restless legs syndrome and periodic limb movement disorders: insomnia; and circadian rhythm sleep disorders. Where appropriate, previously published practice parameters papers are cited and discussed. The relevant published peer-reviewed literature used as the basis for critical decisions was compiled into accompanying evidence tables and is analyzed in the text. In the section on the assessment of sleep apnea syndrome, options for estimating pretest probability to select high risk patients are also reviewed. Sleep-testing procedures other than standard polysomnography are also addressed (daytime polysomnography, split-night studies, oximetry, limited full respiratory recordings, and less-than-full respiratory recording) and treatment-related follow-up studies are discussed.