Framed in the context of researching influences on physical activity and actually working with individuals and groups seeking to initiate, increase or maintain physical activity, the purpose of this review is to present the model of Intentional Development as a multi-theoretical approach to guide research and applied work in physical activity. Elements within the model include ecological influences and developmental areas of attitude, attention, action, and adherence. In breaking down influences on physical activity into these elements, researchers and practitioners can closely examine and understand the key areas that need focus for initiating, increasing, and maintaining lifelong physical activity. (Contains 1 figure.)