Polysomnography refers to the continuous monitoring of multiple neurophysiological and cardiorespiratory variables, usually over the course of a night, to study normal and disturbed sleep. Electroencephalographic, electrooculographic and electromyographic channels provide the basis for staging the recording into successive epochs of wakefulness and various non-Rapid-Eye-Movement (REM) and REM sleep. Detection of airflow at the nose and mouth by means of thermistors, and analysis of breathing patterns recorded from sensors placed around the ribcage and abdomen, sound recordings, the electrocardiogram and the combined application of several other measurement techniques allow the assessment of normal and abnormal physiological events in relation to sleep structure. Polysomnography has an important role in the diagnosis of disorders of sleep and wakefulness as well as in the assessment of treatment effects and in scientific investigations. Developments in digital signal processing have contributed to its widespread application in clinical practice and research.