Actigraphy is a methodology for recording and analyzing activity (movement) from small, computerized devices worn on the body. Published reports on the reliability and validity of actigraph measures, although not comprehensive, generally indicate that sleep estimated by scoring algorithms is relatively consistent with PSG-scored sleep for normal individuals across the lifespan and for some patient groups. Accuracy is often greatly decreased when sleep is disordered or disrupted. Although actigraphy maybe suitable for documenting and evaluating some sleep disorders, its role in clinical diagnosis is limited. Actigraphy is a useful methodology for investigating group differences, sleep-pattern variations over time, and the effects of behavioral or treatment interventions. Controlling artifacts is extremely important, and using some form of daily log is essential for documenting events. The recording period should be long enough to provide reliable measures and to capture important variations across time.