Since the early 1990s, the field of childhood obesity has known that children gain 3-5 times more weight during their 3-month summer vacation than they do during the entire 9-month school year. Evidence shows that youth from low-income households are especially vulnerable to accelerated BMI gain during the summer. The cohort study we seek to extend in this renewal R01 is What’s UP (Undermining Prevention) with Summer. This is the largest and most diverse study of elementary-age children designed to understand accelerated BMI gain during the summer. In this R01, children are measured on key obesity-related health behaviors (i.e., activity, sleep, screen time, time spent sedentary, and diet) during school year (April/May) and in the summer (July). What’s UP with Summer advances the field of accelerated BMI gain during the summer by 1) measuring the SAME children during school and again during summer over multiple years using a within-person design; 2) measuring height/weight at the beginning and end of each summer over time to examine changes in BMI gain during the school year and compare this to BMI gain during the summer; and 3) leveraging rich time/date stamped 24hr accelerometry data and combining this with daily time diary recordings of where children are, what times they are there, when and what they eat, and who they are with. Accelerated BMI gain is not a phenomenon that occurs only in elementary-age children. Suggestive, yet limited, data suggest that middle and high school- age youth also exhibit accelerated BMI gain during the summer. Unlike elementary-age children, important differences in autonomy and decision making occur during these formative adolescent years. The major gaps in the science are few studies measure key health behaviors during the school and summer in middle and high school youth and limited information exists about the social and setting contextual influences on middle and high school health behaviors over the summer and how they impact changes in BMI. Without information collected as proposed in this renewal application, the ability to design effective public health interventions to address obesity during the summer for adolescents is severely limited. In this R01 we will compare longitudinal changes in BMI z-scores and health behaviors during school and summer and from elementary school to high school, identify individual, peer, family/home, neighborhood, and school/community influences on BMI z-scores and health behaviors during school and summer from elementary school to high school, and qualitatively explore changes in adolescents’ BMI and health behaviors during the summer and school. This project is significant because it will extend a well-established existing cohort to identify the impact of an understudied timeframe (summer) associated with accelerated BMI gain during a developmental period for which no information exists (adolescence). This project is innovative because it will capture information on the sa...