Physical activity is an important determinant of health in children and adults. Assessment of physical activity is therefore an important factor in the promotion of health and in several childhood and adulthood pathological situations. Physical activity can be estimated by various methods: pedometry, heart rate monitoring, questionnaires, the doubly labelled water method, and accelerometry. The choice of the type of device depends on the objectives of the clinician or researcher. Accelerometry currently represents the best objective method for measuring physical activity and is widely used in clinical or epidemiological studies. The purpose of this paper is to help practitioners and researchers to make better decisions when using accelerometry as a device for measuring physical activity measurement in order to obtain the most accurate and comparable information.