Researchers interested in evaluating the relationship between physical activity and health requires accurate measures of physical activity. Physical activity is a complex behavior and therefore it is difficult to measure. To describe levels and patterns of physical activity, information regarding the duration, frequency and intensity is needed. The most common way to assess physical activity is by using self report instruments, i.e. (subjective) questionnaires. However, they are hampered by low validity due to recall bias such as social desirability, differential bias between different population groups and reactivity. Objective monitoring can overcome many of these limitations. A number of objective methodologies exist. The simplest and cheapest option is pedometry. A pedometer counts the number of steps taken but it cannot be used to describe the intensity, duration and frequency of the activity performed, which limits the use of it. Another possible objective option is using an indirect estimate of physical activity; heart rate monitoring. The shortcoming is that it needs to be individually calibrated to give accurate measures, which is time and labor consuming. Furthermore, the heart rate is highly influenced by other biological factors such as stress and anger. A third option is the use of accelerometry. Accelerometry overcomes many cultural, linguistic and recall limitations of physical activity measurement and can assess total physical activity as well as time spent in different intensities and sedentary time. The technological development is currently intensive. A large amount of studies have shown strong associations between accelerometry measured physical activity and different health outcomes. The cost for implementation of the methodology is rapidly lowering.