BACKGROUND: Involving men and boys as both users and supporters of Family Planning (FP) is now considered essential for optimising maternal and child health outcomes. Evidence on how to engage men and boys to meet FP needs is therefore important. OBJECTIVES: The main objective of this review was to assess the strength of evidence in the area and uncover the effective components and critical process- and system-level characteristics of successful interventions. SEARCH METHODS: We searched nine electronic databases, seven grey literature databases, organisational websites, and the reference lists of systematic reviews relating to FP. To identify process evaluations and qualitative papers associated with the included experimental studies, we used Connected Papers and hand searches of reference lists. SELECTION CRITERIA: Experimental and quasi-experimental studies of behavioural and service-level interventions involving males aged 10 years or over in low- and middle-income countries to increase uptake of FP methods were included in this review. DATA COLLECTION AND ANALYSIS: Methodology was a causal chain analysis involving the development and testing of a logic model of intervention components based on stakeholder consultation and prior research. Qualitative and quantitative data relating to the evaluation studies and interventions were extracted based on the principles of 'effectiveness-plus' reviews. Quantitative analysis was undertaken using r with robust variance estimation (RVE), meta-analysis and meta-regression. Qualitative analysis involved 'best fit' framework synthesis. RESULTS: We identified 8885 potentially relevant records and included 127 in the review. Fifty-nine (46%) of these were randomised trials, the remainder were quasi-experimental studies with a comparison group. Fifty-four percent of the included studies were assessed as having a high risk of bias. A meta-analysis of 72 studies (k = 265) showed that the included group of interventions had statistically significantly higher odds of improving contraceptive use when compared to comparison groups (odds ratio = 1.38, confidence interval = 1.21 to 1.57, prediction interval = 0.36 to 5.31, p < 0.0001), but there were substantial variations in the effect sizes of the studies (Q = 40,647, df = 264, p < 0.0001; I 2 = 98%) and 73% was within cluster/study. Multi-variate meta-regression revealed several significant intervention delivery characteristics that moderate contraceptive use. These included community-based educational FP interventions, interventions delivered to women as well as men and interventions delivered by trained facilitators, professionals, or peers in community, home and community, or school settings. None of the eight identified intervention components or 33 combinations of components were significant moderators of effects on contraceptive use. Qualitative analysis highlighted some of the barriers and facilitators of effective models of FP that should be considered in future practice and research. AUTHORS' CONCLUSIONS: FP interventions that involve men and boys alongside women and girls are effective in improving uptake and use of contraceptives. The evidence suggests that policy should continue to promote the involvement of men and boys in FP in ways that also promote gender equality. Recommendations for research include the need for evaluations during conflict and disease outbreaks, and evaluation of gender transformative interventions which engage men and boys as contraceptive users and supporters in helping to achieve desired family size, fertility promotion, safe conception, as well as promoting equitable family planning decision-making for women and girls.