Government and educational priorities place importance on young people of secondary school age being active, having their voices heard, and participating in their community. This paper explores an understanding of the role of agency in young people's lives and how the concept is developing. Young people who perceive themselves as having agency may feel they have the ability to change something about themselves or their environment for a valued goal; this may increase their resilience and adaptability to life challenges. A systematic review of agency-related literature revealed little consistency and coherence in definitions and components of agency, particularly regarding young people. Empirical research used realist grounded theory to analyse discussion of three focus groups consisting of 11 young people aged 13 to 17 involved in active school and community projects in both rural and urban areas from a large county in the UK. Findings indicate how certain psychological factors relate over time between a young person's thoughts and feelings, their structural context and their positive interpersonal relationships to support their sense of being an agent. This article argues that focus on young people's agency is a key thread linking many areas of educational practice to empower young people. Implications for practice with young people, professionals and families are discussed, including use of a schema of the findings to guide assessment and intervention.