Aim: I describe the original contribution my work has made to understanding individual, social and environmental influences on physical activity among young people in Scotland. The work I present is based on six publications from five studies. Design/Methodology/Approach: Two studies used quantitative methods and include a 5-year longitudinal study and a cross-sectional pupil and school-level study. Data were collected through self-administered questionnaires. Three studies used qualitative methods including focus group discussions, photography and descriptive writing. All studies involved adolescents (10-15 years) attending schools in Scotland. Key Findings: My longitudinal evidence showed significant gender and developmental effects through early-mid adolescence, particularly at the primary-secondary school transition. While boys reported higher levels of physical activity than girls at all ages, baseline physical activity was a significant predictor of later activity levels for both genders. The importance of social influences was evident across my research. Survey data showed boys to report higher peer support, parental support and independent play than girls. Among both genders, peer, paternal and maternal support decreased with age, whereas independent play increased. Time spent with friends was particularly important and was further highlighted in qualitative findings. Being with friends was a benefit of active travel to school, walking, and general physical activity. Other factors influencing physical activity included proximity and access to local facilities, and the school environment. Most notable was the importance of cost and value for money. In general, pupils were satisfied with school facilities, but felt time for physical education (PE) could be increased. Among adolescent girls, survey data showed the best predictor of physical activity to be hours allocated to PE in fourth year of secondary school. Provision of PE and extra-curricular clubs are likely to be of greater importance to girls’ participation than schools facilities. Longitudinal data showed girls reporting lower levels of perceived competence, self-esteem and physical self-worth than boys at each age. Furthermore, girls’ physical perceptions decreased markedly over time, whereas among boys, only perceived competence decreased, while global esteem increased. Influences on physical activity participation were shown to be context specific, e.g. pupils reported not having bicycle storage as a barrier to cycling to school. Walking was a popular activity among girls, although areas in which walking took place, and reasons for walking could differ between geographical locations. Conclusions: In line with the socio-ecological model, my findings demonstrate the complex interaction of individual, social and environmental factors on physical activity participation among young people. They highlight the need for all sectors to work together in developing the most suitable interventions, considering the activity, its context and the population being targeted.