Participating CAMHS teams or sites will be randomised to intervention or control groups; with the intervention group receiving care according to the model of managed transition and the control group receiving treatment as usual. Clinicians in the intervention arm will benefit from initial trainingand an evidence-based, decision-making process regarding transition need. The model of managed transition will support current service provision and provide a foundation on which good quality transition practices can be built, if these are not already in place.

In 9 study sites from eight European countries, 1000 service users will be recruited. Out of them, 200 will participate in the intervention arm of the study and 800 will be included in the control arm.

During the transition phase, service users recruited to the intervention group will take part in a baseline assessment, comprising of an interview and various online assessments, including the Transition Readiness and Appropriateness Measure (TRAM), a decision support tool. A Research Assistant (RA) will communicate the TRAM findings to the CAMHS clinician or team, offering to explain the findings at a face-to-face meeting. The clinician will be expected to communicate the TRAM results to the service user. If the CAMHS clinician refers the young person to AMHS, and it is appropriate, the RA will offer to explain the TRAM findings to the AMHS clinician. Baseline assessments will also be carried out in the control group. In contrast to the intervention group, corresponding results won’t be relayed to clinicians. Treatment will be continued as usual.

Both groups will then be followed up for 27 months, assessments happening at three time points, at nine month intervals. CAMHS and AMHS clinicians responsible for the recruited service users will be asked to complete a short online assessment at each time point too. At the CAMHS site this will happen at baseline only unless the young person is retained at CAMHS.

The MILESTONE study will cause no disruption to clinical care and only minimal additional work for clinicians and therapists.