This paper explores socio-ecological factors and their interplay that emerge from a qualitative study and influence adolescents' physical activity and sport participation. A total of 13 boys and 7 girls active and inactive adolescents, from years 12 and 13 and different types of school (state and private), participated in semi-structured interviews. It followed a purposeful sampling selection from participants who participated in a previous larger study on physical activity levels. Findings highlight the influence of personal and social interplay, such as perceived competence, obesity and teasing from peers, and new social demands and personal preferences in adolescents' (dis)engagement in physical activity and sport. They also stand out how social and environmental factors affect their participation, such as family, boy/girlfriends, physical education and rural/urban places of residence. These factors appear to have an important impact on the construction of (in)active identities during late adolescence. Policy makers, exercise professionals and educators should be aware of these factors in order to improve health promotion strategies and social policies. Some recommendations are also indicated in the paper.