BackgroundThe mental health and well-being of care-experienced children and young people remains a concern. Despite a range of interventions, the existing evidence base is limited in scope, with a reliance on standalone outcome evaluations which limits understanding of how contextual factors influence implementation and acceptability. The Care-experienced cHildren and young people’s Interventions to improve Mental health and wEll-being outcomes Systematic review (CHIMES) aimed to synthesise evidence of intervention theory, outcome, process and economic effectiveness. This paper reports the process evaluation synthesis, exploring how system factors facilitate and inhibit implementation and acceptability of mental health and wellbeing interventions for care-experienced children and young people. MethodsSixteen databases and 22 websites were searched between 2020 and 2022 for studies published from 1990 and May 2022. This was supplemented with contacting experts in the field, citation tracking, screening of relevant systematic reviews and stakeholder consultations. We drew on framework synthesis of qualitative data and incorporated a systems lens, taking account of contextual influences across socio-ecological domains. Quality appraisal assessed reliability and usefulness. Confidence in synthesised findings was assessed with the GRADE-CERQual tool. We report the review in accordance with relevant elements of both the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA), and the Enhancing transparency in reporting the synthesis of qualitative research (ENTREQ) checklist. ResultsSearches retrieved 15,068 unique study reports, and 23 of these were eligible for process evaluation synthesis, reporting on sixteen interventions. Studies were published between 2003 and 2021. Nine interventions were from the UK and Ireland, six interventions were from the USA, and one was from Australia. They were largely classified as interpersonal, where the aim was to modify carer-child relationships. Five key context factors were identified that supported and prohibited intervention delivery: (1) lack of system resources; (2) intervention burden, which encompasses the time, cognitive, and emotional burden associated with implementation and participation; (3) interprofessional relationships between health and social care professionals; (4) care-experienced young people’s identity; and (5) carer identity. ConclusionWe identified several supportive and restrictive factors across social and health care systems that may impact intervention implementation and acceptability. Key implications include: the importance of involving diverse stakeholders in intervention development and delivery; the need to better resource and support those involved in interventions, particularly training and support for carers; and ensuring future evaluations integrate process evaluations in order to optimise interventions.