Background Risk assessment is a key process when a child or adolescent presents at risk for self-harm or suicide in a mental health crisis or emergency. Risk assessment by a healthcare professional should be included within a biopsychosocial assessment. However, the predictive value of risk-screening tools for self-harm and suicide in children and adolescents is consistently challenged. A review is needed to explore how best to undertake risk assessment and the appropriate role for tools/checklists within the assessment pathway. Aims To map research relating to risk assessment for child and adolescent mental health and to identify features that relate to a successful risk assessment. Objectives To review factors within the clinical encounter that impact upon risk assessments for self-harm and suicide in children and adolescents: to conduct a realist synthesis to understand mechanisms for risk assessment, why they occur and how they vary by context to conduct a mapping review of primary studies/reviews to describe available tools of applicability to the UK. Data sources Databases, including MEDLINE, PsycINFO^®, EMBASE, CINAHL, HMIC, Science and Social Sciences Citation Index and the Cochrane Library, were searched (September 2021). Searches were also conducted for reports from websites. Review methods A resource-constrained realist synthesis was conducted exploring factors that impact upon risk assessments for self-harm and suicide. This was accompanied by a mapping review of primary studies/reviews describing risk-assessment tools and approaches used in UK child and adolescent mental health. Following piloting, four reviewers screened retrieved records. Items were coded for the mapping and/or for inclusion in the realist synthesis. The review team examined the validity and limitations of risk-screening tools. In addition, the team identified structured approaches to risk assessment. Reporting of the realist synthesis followed RAMESES guidelines. Results From 4084 unique citations, 249 papers were reviewed and 41 studies (49 tools) were included in the mapping review. Eight reviews were identified following full-text screening. Fifty-seven papers were identified for the realist review. Findings highlight 14 explanations (programme theories) for a successful risk assessment for self-harm and suicide. Forty-nine individual assessment tools/approaches were identified. Few tools were developed in the UK, specifically for children and adolescents. These lacked formal independent evaluation. No risk-screening tool is suitable for risk prediction; optimal approaches incorporate a relationship of trust, involvement of the family, where appropriate, and a patient-centred holistic approach. The objective of risk assessment should be elicitation of information to direct a risk formulation and care plan. Limitations Many identified tools are well-established but lack scientific validity, particularly predictive validity, or clinical utility. Programme theories were generated rapidly from a survey of risk assessment. Conclusions No single checklist/approach meets the needs of risk assessment for self-harm and suicide. A whole-system approach is required, informed by structured clinical judgement. Useful components include a holistic assessment within a climate of trust, facilitated by family involvement. Study registration This study is registered as PROSPERO CRD42021276671. Funding This award was funded by the National Institute for Health and Care Research (NIHR) Health and Social Care Delivery Research programme (NIHR award ref: NIHR135079) and is published in full in Health and Social Care Delivery Research; Vol. 12, No. 1. See the NIHR Funding and Awards website for further award information.