Motor activity as a diagnostic parameter has become an important feature in many fields of medicine and psychology. The concept of mobility and immobility implies the assumption that mental and behaviour disorders involve abnormal activity that can be measured to characterise the disorder itself, to diagnose its presence and to document the impact of treatment. In sleep research, activity monitoring by wrist actigraphs has proven its usefulness as an efficient method to assess the rest-activity cycle over long time periods and to estimate sleep-related features such as sleep efficiency and total sleep time. But like many other techniques and devices, activity monitoring has some limitations and drawbacks. This paper describes the basic features of wrist actigraphy in measuring nocturnal and daytime motor activity.