This systematic review presents research on the physical activity levels of preschool-aged children (aged 2-6 years). Thirty-nine primary studies (published 1986-2007) representing a total of 10,316 participants (5236 male and 5080 female), from seven countries are described and the physical activity behaviors of this population are considered in accordance with the National Association for Sport and Physical Education (NASPE) physical activity guidelines for preschoolers. Upon review of the evidence, it is apparent that nearly half of preschool-aged children do not engage in sufficient physical activity. Current recommendations suggest a minimum of 60 min of physical activity per day; only 54% of participants throughout the studies achieved this. Furthermore, as with other age groups, boys participate in considerably more physical activity than girls. It is clear from this systematic review that nearly half of children studied are not meeting the recommended guidelines for physical activity. Therefore, effective interventions that promote and foster physical activity in children are necessary, especially in females. However, a more objective physical activity guideline for preschoolers is necessary; measurement of activity needs to become more unified to compare and track activity more effectively. (Contains 1 table.)