The theoretical models derived from lifecourse epidemiology track exposures across the lifecourse and create an understanding of their accumulative effects on health trajectories and health outcomes. This model has been around for some years. The understanding is that disease in adulthood is often brought about by the cumulative effects of many exposures, incidents and behaviours, over a long period of time, starting as far back as infancy – accumulative risk, critical and sensitive periods. What makes people active and what motivates people to be active is therefore presented in this chapter. But what instruments best capture these changes? What factors should you use to inform your choice? How does physical activity change as we age and how does it affect measurement?