AbstractReviews around interventions to improve shared decision making (SDM) for child and youth mental health have produced inconclusive findings on what approaches increase participation. Importantly, the previous reviews did not explore the use of theory, as well as mechanisms of change (intervention functions) and active units of change (behaviour change techniques). The aim of this review was to explore these factors and ascertain how, if at all, these contribute to SDM. Five databases were searched up until April 2020. Studies met inclusion criteria if they were: (a) an intervention to facilitate SDM; (b) aimed at children, adolescence, or young people aged up to 25, with a mental health difficulty, or their parents/guardians; and (c) included a control group. Data were extracted on patient characteristics, study design, intervention, theoretical background, intervention functions, behaviour change techniques, and SDM. Quality assessment of the studies was undertaken using the Effective Public Health Practice Project (EPHPP) quality assessment tool. Eight different interventions met inclusion criteria. The role of theory to increase SDM remains unclear. Specific intervention functions, such as ‘education’ on SDM and treatment options and ‘environmental restructuring’ using decision aids, are being used in SDM interventions, as well as ‘training’ for clinicians. Similarly, behaviour change techniques linked to these, such as ‘adding objects to the environment’, ‘discussing pros/cons’, and clinicians engaging in ‘behavioural practice/rehearsal’. However, as most studies scored low on the quality assessment criteria, as well as a small number of studies included and a low number of behaviour change techniques utilised, links between behaviour change techniques, intervention functions and increased participation remain tentative. Intervention developers and clinicians may wish to consider specific intervention functions and behaviour change techniques to facilitate SDM.