Research has shown that the brain undergoes substantial development during human adolescence, particularly in regions associated with cognitive control and social cognition. Successful transition to adulthood requires the refinement and integration of these processes. The studies in this thesis aimed to investigate how interactions between social cognition, motivational-affective processing, and cognitive control change over adolescent development, and how this is influenced by individual differences in affective reactivity and genetics. The studies described in the first two experimental chapters examined the development of several aspects of cognition and how this is affected by a common genetic polymorphism associated with dopaminergic variation (COMT). Chapter 2 investigated the development of social, relative to non-social, working memory, and how this is moderated by COMT. Chapter 3 explored developmental changes in the association of COMT with the processing of self- generated information, and self-reported trait anxiety. Together, these studies demonstrate the importance of considering genetic variation from a developmental perspective. Adolescence is characterised by changes in learning and decision-making, processes that require the coordination of motivational-affective processing and cognitive control, and this is the subject of Chapters 4 and 5. Chapter 4 used a computational reinforcement learning paradigm to investigate how adolescents, as compared to adults, learn from reward versus punishment, and from counterfactual feedback about their decisions. In Chapter 5, developmental changes in social reward sensitivity, and whether this is related to variation in social anxiety, were investigated. This thesis also investigated how variation in cognitive control and socio-affective processing relates to the development of affective disorders. While Chapters 3 and 5 used self-report measures to explore the relationship between affective reactivity and development of cognitive control and socio-affective processing, Chapter 6 examined how interactions between these processes relate to the onset of adolescent depression in a one-year longitudinal sample of high-risk adolescents.