This is the protocol for a Campbell systematic review. The objectives are as follows. (1) Examine whether secondary and tertiary interventions delivered outside of the criminal justice system are effective at countering the cognitive and behavioural radicalisation of children and adolescents by synthesising evidence relating to relevant primary and secondary outcomes of effectiveness. (2) Examine whether secondary and tertiary interventions delivered outside of the criminal justice system are being implemented as intended by synthesising evidence that captures how interventions are implemented, considering whether they are implemented as expected or in ways that align with their underlying logic. (3) Identify those implementation factors (facilitators and barriers) and moderators that impact how interventions working with children and adolescents are delivered.