For more than a decade there has been growing concern about global reductions in physical activity and increases in sedentary behaviours. Initially, it was unclear whether children would be protected from this trend. Perhaps children's playfulness and associated activity levels would act as a protective factor. There is now compelling evidence that children's activity levels are quite sensitive to environmental factors. For example, a recent US study of activity levels in preschoolers concluded that "...the characteristics of the school have a much greater influence on a child's activity level while in school than do the child's personal demographic characteristics" (Pate et al. 2004). There is also clear evidence that children's freedom to engage in active play, particularly outdoors, has diminished over the last generation (Clements, 2004). In this chapter, we examine some of the factors in young children's environments that influence levels of physical activity. Our main focus is on the physical characteristics of formal child care environments and to a lesser extent, school playgrounds. We examine the role of time, space, loose objects, risk-taking/safety and outdoor pedagogy in the context of children's play environments.