AbstractWhen young people reach the upper age limit of child and adolescent mental health services (CAMHS), care should be transferred to an adult mental health service (AMHS) if they require ongoing support. However, many young people experience a significant disruption of their care during this transition, whilst others may fail to transition at all. Currently, there is no systematic appraisal of the international evidence regarding the outcomes of young people after transition. A systematic review was conducted which aimed to synthesise and review the existing research regarding outcomes after transition. We searched six databases from their inception until December 2017 for research relating to either the mental health or service use outcomes of young people after reaching their CAMHS age boundary. Results were synthesised narratively. The initial searches identified 18,287 papers, of which 213 were screened on full text. 13 papers were included in the review, representing 10 cohorts of young people who crossed the transition age boundary. No studies contained extractable data on mental health outcomes following transition, and therefore, this review focused only on service use outcomes. Results showed a quarter of young people transitioned to AMHS, with the other young people experiencing varied outcomes after leaving CAMHS and multiple transitions during this time. This review provides evidence for the varying service use outcomes of young people after reaching the upper age limit of CAMHS. However, longitudinal research into long-term outcomes is lacking, in addition to research regarding the mental health and functioning outcomes of young people following transition.Protocol registration The protocol for this systematic review has been registered with PROSPERO, ID number CRD42018085916.