The prevalence of childhood obesity in the U.S. has increased dramatically over the past three decades, and the trend has become evident even in the youngest children. Approximately 8% of infants and toddlers have high weight-for-length, which increases risk for developing obesity and other health problems in childhood and beyond. Thus, leading health organizations have identified prevention of obesity in young children as a critical public health challenge. Low levels of physical activity and high levels of sedentary behavior may contribute to the development of excessive fatness in young children, but these relationships have not been fully explored. In children ages 3 and above, accelerometry has been used widely to measure physical activity and sedentary behavior. However, it rarely has been used to measure physical activity levels in infants and toddlers, and then only in cross-sectional studies. No previous study has used accelerometry as an objective measure of physical activity in young children as they develop from infancy to preschool age. Accordingly, little is known about the factors associated with the development of physical activity behavior in very young children, and little is known about the influence of physical activity and sedentary behavior, measured objectively, on development of weight status during the transition from infancy to age 3. The first aim of this study is to describe physical activity and sedentary behavior in young children as they develop from infancy to preschool age. The second aim is to describe the longitudinal associations of weight status with physical activity and sedentary behavior as young children develop from infancy to preschool age. The proposed investigation will employ a longitudinal, observational study design. Participants will be 160 children and their biological mothers living in Columbia, South Carolina. Participants will be recruited through childcare centers, pediatrics practices, churches, and community organizations in Richland County. For each participating child, measurements will be taken at 6, 12, 18, 24, 30, and 36 months of age. At each time point, each child’s physical activity, sedentary behavior, weight status and motor developmental status will be measured objectively, and each child’s mother will complete a survey to assess demographic, social and physical environmental factors; gross motor milestones; parenting practices related to physical activity and sedentary behavior; and dietary practices. Childcare center directors will complete a survey annually to assess center characteristics, and the Environment and Policy Assessment and Observation (EPAO) will be administered in the child’s classroom annually. The proposed research will be significant because it will markedly expand our knowledge of the relationships among physical activity, sedentary behavior and development of adiposity during the first three years of life. The proposed investigation will be innovative because...