OBJECTIVES: To determine the effectiveness of primary health care relevant interventions to prevent and treat tobacco smoking in school-aged children and adolescents. METHODS: This systematic review considered studies included in a prior review. We adapted and updated the search to April 2015. Titles, abstracts and full-text articles were reviewed in duplicate; data extraction and quality assessments were performed by one reviewer and verified by another. Meta-analyses and pre-specified sub-group analyses were performed when possible. PROSPERO #CRD42015019051. RESULTS: After screening 2118 records, we included nine randomized controlled trials. The mostly moderate quality evidence suggested targeted behavioural interventions can prevent smoking and assist with cessation. Meta-analysis showed intervention participants were 18% less likely to report having initiated smoking at the end of intervention relative to controls (RR 0.82; 95% CI 0.72, 0.94); absolute effect 1.92% for smoking initiation, NNT 52 (95% CI 33, 161). For cessation, meta-analysis showed intervention participants were 34% more likely to report having quit smoking at the end of intervention relative to controls (RR 1.34; 95% CI 1.05, 1.69); absolute effect 7.98% for cessation, NNT 13 (95% CI 6, 77). Treatment harms were not mentioned in the literature and no data were available to assess long-term effectiveness. CONCLUSION: Primary care relevant behavioural interventions improve smoking outcomes for children and youth. The evidence on key components is limited by heterogeneity in methodology and intervention strategy. Future trials should target tailored prevention or treatment approaches, establish uniform definition and measurement of smoking, isolate optimal intervention components, and include long-term follow-up