This is the protocol for a Campbell systematic review. The review will address the following research questions: What is the evidence on the effects of adult mentoring programmes in reducing anti-social, violent and offending behaviour in children aged under 18 years? Are these effects sustained after the end of mentoring? Which aspects/features of adult mentoring programmes promote the reduction of anti-social, violent and criminal behaviour in children aged under 18 years? What are the hindering factors/barriers that affect the successful implementation of adult mentoring programmes in children aged under 18 years? What are the supporting factors/facilitators that contribute to the successful implementation of adult mentoring programmes in children aged under 18 years? What is the evidence on programme costs and incremental cost effectiveness? (The incremental (or marginal cost) is the cost of providing the intervention over and above the cost of usual services).