The physical activity behavior of young preschool-aged children is different from that seen in older children and adults, due in part to biological, psychosocial and cognitive immaturity. This paper: (a) provides an overview the relevant development of young children in relation to physical activity; and (b) outlines the relationship between child development, play, and physical activity in young children. Understanding, assessing and promoting physical activity in young children should include identification of contextual factors such as developmental stage and aspects of play.