||This article illustrates how a life-course perspective can be infused more fully into the research field of physical activity promotion. A life-course perspective is particularly promising in connecting, organizing, and supplementing current knowledge and can potentially stimulate and direct future research and intervention efforts by using a time-sensitive, opportunistic, and developmental approach. The first section summarizes this approach into five key life-course principles including human agency, linked lives, time and place, life-span development, and timing. The second section takes a closer look at three time-based components: trajectories, transitions, and turning points. The final section highlights some of the implications of a life-course perspective for research methods and interventions, especially among children and older adults.