Sleep research has evolved to sleep medicine over the past 20 years. Sleep laboratories are now established in many places in order to diagnose and treat sleep disorders. The most prevalent sleep disorder is insomnia, but this is rarely investigated in a sleep laboratory. Second most prevalent are sleep-related breathing disorders, with obstructive sleep apnea as the best known disorder. A sleep laboratory provides methods for diagnosing this and other sleep disorders with the use of cardiorespiratory polysomnography. In addition, daytime testing of sleepiness is used to check the effect of treatment on daytime function and performance. Cardiorespiratory polysomnography uses digital equipment with defined requirements on sleep signals, respiratory, movement, and cardiovascular signals. Computer-based polysomnography tries to support the sleep expert with automated analysis of sleep, respiration, and movement. These methods did not succeed in replacing visual evaluation methods until now. Currently, the definitions for sleep stages are being reassessed. The revised definitions may improve the basis for the development of analysis algorithms, which can then achieve a better validity compared with expert evaluation. In addition, new developments in the field of sensor technology make use of miniaturization and noninvasive signal assessment in order to improve patient sleep comfort during the sleep study.