Advanced ambulatory systems that measure aspects of overt human behavior during normal daily life have become feasible, owing to developments in data recording and sensor technology. One such instrument is the Activity Monitor (AM). This paper provides a technical description of the AM and information about its validity and current applications. The AM is based on ambulatory accelerometry, the aim of which is to assess postures and motions for long-term (> 24-h) measurement periods during normal daily life. Accelerometers are attached to the thighs, trunk, and lower arms, and signals are continuously stored in a digital portable recorder. In the postmeasurement analysis, postures and motions are detected by means of custom-made software programs. Validity studies performed on different populations showed high agreement scores between the computerized and automatic AM output and the visually analyzed video recordings. The AM has so far been applied in rehabilitation, psychophysiology, and cardiology but has many possibilities in behavioral research.