["Health Expectations, Volume 29, Issue 2, April 2026. ", "\nABSTRACT\n\nBackground\nAdolescent involvement in health research has numerous benefits for the research, researchers, and the adolescents involved. However, evidence on the impacts of adolescent involvement in health research is anecdotal and subjective, relying on informal reflections rather than prospectively planned and rigorous evaluations. These limitations of the current evidence base highlight the need for more structured and prospectively planned evaluations of adolescent involvement in health research. To achieve this, we explored how adolescent co‐researchers involved in a guideline development project experienced their involvement, what facilitated their sustained participation, what outcomes resulted, and how specific engagement practices may have contributed to these outcomes.\n\n\nMethods\nThis evaluation was co‐designed and led by trained adolescent co‐researchers. Semi‐structured interviews were conducted with five adolescent co‐researchers involved in a guideline development project. Reflexive thematic analysis was conducted to analyse the data. Emerging themes were mapped onto the Youth Engagement Framework. To build a theory of change, we used the Public Involvement Impact Assessment Framework to trace how initial motivations, engagement practices, and contextual barriers contributed to observed outcomes.\n\n\nResults\nFindings are organised around four key themes: initiators, qualities of engagement process, sustainers, and outcomes of the involvement process. Adolescent co‐researchers reported being motivated by opportunities for skill development, career growth, and roles aligned with their interests, and highlighted the role of supportive involvement practices in sustaining their involvement. They shared some predominantly positive outcomes from their involvement, including enhanced research knowledge and skills, co‐authorship opportunities, financial compensation, and strengthened relationships with the research team.\n\n\nConclusion\nAdolescent engagement is shaped not just by individual motivation, but also by social and institutional factors, indicating the need for system‐level changes to enable meaningful and equitable involvement. To maximise benefits, researchers should tailor involvement processes to adolescents' needs, supported by a clear theory of change.\n\n\nPublic and Patient Involvement\nThis evaluation was co‐designed with adolescents and led by a trained adolescent co‐researcher. Three young people (aged 17–24) were involved at different stages of the study. One acted as a consultant during the study design phase, helping to define the evaluation framework and refine interview guides. The others were hired as co‐researchers to support data collection and analysis. The lead adolescent co‐researcher conducted interviews, proofread transcripts, supported coding and theme development, and contributed to manuscript drafting.\n"]