Transition to adulthood is the period of onset of most of the serious mental disorders that disable or kill in adult life. Current service configuration of distinct Child and Adolescent Mental Health (CAMHS) and Adult Mental Health (AMHS) Services is considered the weakest link where the care pathway should be most robust. Transition-related discontinuity of care is a major socioeconomic and societal challenge for the EU. The MILESTONE project is an EU-wide study determining care gaps in current services across diverse healthcare systems and robustly evaluating an innovative transitional care model. In ten high-quality work packages we will map current services and transitional policies across EU; develop and validate transition-specific outcomes measures; conduct a longitudinal cohort study of transition process and outcomes across eight EU countries; develop and test, in a cluster-randomised trial, the clinical and cost-effectiveness of an innovative transitional care model; create clinical, organisational, policy and ethics guidelines for improving care and outcomes for transition age youth; and develop and implement training packages for clinicians across EU. The project will provide robust evidence for the most cost-effective way to meet the as-yet-unmet need of young people who fall through the CAMHS-AMHS divide; facilitate the development of integrated models of care and function; improve health care outcomes and system efficiencies; and ensure take-up of best practice. The project has active and intensive participation of young people, carers, advocacy groups and key stakeholders and involves two SMEs, Concentris and HealthTracker. Findings from the project will transform mental health care in EU for young people. Our results will assist policy makers in making informed and evidence-based decisions for improving health systems, enhancing patient outcomes, quality of life, service satisfaction, and improving health status at individual and population levels.