In many areas of research and medicine, objective data describing an individual's ambulatory function are sought as useful indicators of that person's condition. Normally, detailed measurements are taken over short periods of time within a controlled laboratory setting. To complement this approach, Prosthetics Research Study has developed a small, unobtrusive instrument that continuously records a simple measure--step counts per unit time--as an individual goes about normal daily life. The Step Activity Monitor (SAM) is approximately the size and weight of a pager and is worn at the ankle. It can detect steps with better than 99% accuracy across a wide range of gait styles for adults, children, and large animals. During monitoring, step counts are recorded at consecutive, adjustable time intervals over weeks to months at a time. Recording at 1-min intervals for a minimum of 2 weeks is recommended. Once monitoring is completed, the data are transferred to a computer, and the levels and patterns of step activity can be analyzed. This article provides a detailed description of the SAM, guidelines for use, results of accuracy and reliability testing, case study descriptions demonstrating the ability to measure differences that result from medical interventions or changes in health status, and a discussion of considerations pertinent to long-term monitoring of activity.