The measurement of physical activity and sedentary behaviour using device-based measures (e.g. accelerometers) has traditionally focused on quantifying the volume of activity undertaken. As accelerometers provide date- and time-stamped data, it is possible to examine how and when youth accumulate their activity during the day. These patterns of activity accumulation include the timing, duration, and intensity of activity bouts, and the times of the day when activity occurs. In recent years, there has been some emerging evidence to suggest that the way in which youth accumulate their activity may be important for their health. This presentation will initially highlight the evolution of assessing activity patterns using device-based measures such as accelerometers. It will then focus on the importance of considering patterns of activity accumulation, and provide an overview of research that has examined associations between patterns of activity accumulation with health outcomes. It will finish with a discussion of the implications that understanding how youth accumulate their activity may have for public health.