ABSTRACT Internalizing problems are common, harmful, and increasing amongst adolescent girls -- creating a public health imperative to identify behavioral and biological mechanisms and/or indicators of risk. Social connection, exclusion, and loneliness are well known to affect emergence and recurrence of internalizing symptoms and disorders in adolescent girls, and recent efforts have begun to focus on the specific role of close friendships in these processes. While the quality of close friendships often buffers against risks for mental disorders, in adolescent girls some supportive features of close friendships may also increase risk for depression, anxiety, and self-harm. One important new context in which to consider these processes is the use of digital technology, especially social media, as adolescents extensively use these methods to connect with friends and peers. It is therefore essential to understand how the dynamics of daily online and offline experiences of social connection, social exclusion, and loneliness impact adolescent girls’ mental health. In addition, it is also clear that pubertal development strongly impacts risk trajectories in adolescent girls, likely via concomitant neural and social changes. Neural responses to social exclusion are widely understood to differ in adolescents with depression, anxiety, and self-harm; yet we know very little about how positive aspects of social connection might buffer these individual differences. The Transitions in Adolescent Girls (TAG) study, launched in 2015 (R01 MH107418), was designed to conduct a comprehensive multilevel investigation of the connections between biological and social changes during early-to-mid adolescence, in order to reveal the ways in which these interconnected changes relate to risk for the emergence of a range of mental health problems associated with pubertal development in girls. We enrolled a community sample of N=174 girls (ages 10.0-13.0 years) into a longitudinal study, with 3 waves of data collected every 18 months, including 2 laboratory visits at each wave. The first phase of the TAG study was designed to address the question of whether puberty influences mental health via its impact on neural, self, and social cognitive development). We propose to collect additional waves of data at two more 18-month intervals, bringing the cohort to 16-19 years of age. Critically, we will incorporate an expanded multilevel emphasis on social connection, including both established questionnaires and neuroimaging tasks as well as innovative new methods that leverage mid-to- late adolescents’ use of smartphones. This has the added benefit of extending the project to examine the impact of early-to-mid adolescent biological and psychosocial changes on mid-to-late adolescent social connection and mental health -- a key phase of life for both of these processes. We hypothesize that social connection during mid-to-late adolescence is not only predictive of concurrent and nea...