Transition-related discontinuity of care is a major socioeconomic and societal challenge for the EU. The current service configuration, with distinct Child and Adolescent Mental Health (CAMHS) and Adult Mental Health Services (AMHS), is considered a weak link where the care pathway needs to be most robust. Our aim was to delineate transitional policies and care across Europe and to highlight current gaps in care provision at the service interface. An online mapping survey was conducted across all 28 European Countries using a bespoke instrument: The Standardized Assessment Tool for Mental Health Transition (SATMEHT). The survey was directed at expert(s) in each of the 28 EU countries. The response rate was 100%. Country experts commonly (12/28) reported that between 25 and 49% of CAMHS service users will need transitioning to AMHS. Estimates of the percentage of AMHS users aged under 30 years who had has previous contact with CAMHS were most commonly in the region 20-30% (33% on average).Written policies for managing the interface were available in only four countries and half (14/28) indicated that no transition support services were available. This is the first survey of CAMHS transitional policies and care carried out at a European level. Policymaking on transitional care clearly needs special attention and further elaboration. The Milestone Study on transition should provide much needed data on transition processes and outcomes that could form the basis for improving policy and practice in transitional care.