Obesity rates in 3-5 year-old American children have increased dramatically in recent decades. Concurrent with this alarming trend, time spent by young children in preschool settings has increased and their time spent in unstructured play has decreased. Recent studies have shown that children in preschools are engaged primarily in sedentary activities. Nonetheless, few interventions designed to increase PA or decrease sedentary behavior have been evaluated in preschool children. The proposed investigation tested a multicomponent intervention designed to increase physical activity energy expenditure (PAEE) in 3-5 year-old children. Sixteen preschools were randomly assigned to intervention or control groups. 500 children, in two annual waves of ~250 children, participated in the measurement protocol at baseline and after exposure to the intervention for 9 months. Child-level measures included moderate-to-vigorous PA, sedentary behavior, and PAEE measured by accelerometry; height, weight and waist circumference; and demographics. The intervention consisted of four components: Move IN (physical education and other indoor activity programming), Move OUT (recess and structured outdoor activity), Move to Learn (PA in classroom, academic lessons), and enhancing the social environment to promote PA. The study's intervention coordinator worked with the preschool directors, teachers, and assistant teachers to facilitate intervention implementation by providing training, materials, and ongoing support and feedback. An extensive process evaluation documented the extent to which the intervention was implemented. Effects of the intervention on PA, sedentary behavior, PAEE, and weight status were determined. In addition, the study examined factors that associate with change in physical activity, sedentary behavior, physical activity energy expenditure and body mass index during a school year in preschool children.