BACKGROUND: People living in 'humanitarian settings' in low- and middle-income countries (LMICs) are exposed to a constellation of physical and psychological stressors that make them vulnerable to developing mental disorders. A range of psychological and social interventions have been implemented with the aim to prevent the onset of mental disorders and/or lower psychological distress in populations at risk, and it is not known whether interventions are effective. OBJECTIVES: To compare the efficacy and acceptability of psychological and social interventions versus control conditions (wait list, treatment as usual, attention placebo, psychological placebo, or no treatment) aimed at preventing the onset of non-psychotic mental disorders in people living in LMICs affected by humanitarian crises. SEARCH METHODS: We searched the Cochrane Common Mental Disorders Controlled Trials Register (CCMD-CTR), the Cochrane Drugs and Alcohol Review Group (CDAG) Specialized Register, Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE (OVID), Embase (OVID), PsycINFO (OVID), and ProQuest PILOTS database with results incorporated from searches to February 2020. We also searched the World Health Organization's (WHO) International Clinical Trials Registry Platform and ClinicalTrials.gov to identify unpublished or ongoing studies. We checked the reference lists of relevant studies and reviews. SELECTION CRITERIA: All randomised controlled trials (RCTs) comparing psychological and social interventions versus control conditions to prevent the onset of mental disorders in adults and children living in LMICs affected by humanitarian crises. We excluded studies that enrolled participants based on a positive diagnosis of mental disorder (or based on a proxy of scoring above a cut-off score on a screening measure). DATA COLLECTION AND ANALYSIS: We calculated standardised mean differences for continuous outcomes and risk ratios for dichotomous data, using a random-effects model. We analysed data at endpoint (zero to four weeks after therapy) and at medium term (one to four months after intervention). No data were available at long term (six months or longer). We used GRADE to assess the quality of evidence. MAIN RESULTS: In the present review we included seven RCTs with a total of 2398 participants, coming from both children/adolescents (five RCTs), and adults (two RCTs). Together, the seven RCTs compared six different psychosocial interventions against a control comparator (waiting list in all studies). All the interventions were delivered by paraprofessionals and, with the exception of one study, delivered at a group level. None of the included studies provided data on the efficacy of interventions to prevent the onset of mental disorders (incidence). For the primary outcome of acceptability, there may be no evidence of a difference between psychological and social interventions and control at endpoint for children and adolescents (RR 0.93, 95% CI 0.78 to 1.10; 5 studies, 1372 participants; low-quality evidence) or adults (RR 0.96, 95% CI 0.61 to 1.50; 2 studies, 767 participants; very low quality evidence). No information on adverse events related to the interventions was available. For children's and adolescents' secondary outcomes of prevention interventions, there may be no evidence of a difference between psychological and social intervention groups and control groups for reducing PTSD symptoms (standardised mean difference (SMD) -0.16, 95% CI -0.50 to 0.18; 3 studies, 590 participants; very low quality evidence), depressive symptoms (SMD -0.01, 95% CI -0.29 to 0.31; 4 RCTs, 746 participants; very low quality evidence) and anxiety symptoms (SMD 0.11, 95% CI -0.09 to 0.31; 3 studies, 632 participants; very low quality evidence) at study endpoint. In adults' secondary outcomes of prevention interventions, psychological counselling may be effective for reducing depressive symptoms (MD -7.50, 95% CI -9.19 to -5.81; 1 study, 258 participants; very low quality evidence) and anxiety symptoms (MD -6.10, 95% CI -7.57 to -4.63; 1 study, 258 participants; very low quality evidence) at endpoint. No data were available for PTSD symptoms in the adult population. Owing to the small number of RCTs included in the present review, it was not possible to carry out neither sensitivity nor subgroup analyses. AUTHORS' CONCLUSIONS: Of the seven prevention studies included in this review, none assessed whether prevention interventions reduced the incidence of mental disorders and there may be no evidence for any differences in acceptability. Additionally, for both child and adolescent populations and adult populations, a very small number of RCTs with low quality evidence on the review's secondary outcomes (changes in symptomatology at endpoint) did not suggest any beneficial effect for the studied prevention interventions. Confidence in the findings is hampered by the scarcity of prevention studies eligible for inclusion in the review, by risk of bias in the studies, and by substantial levels of heterogeneity. Moreover, it is possible that random error had a role in distorting results, and that a more thorough picture of the efficacy of prevention interventions will be provided by future studies. For this reason, prevention studies are urgently needed to assess the impact of interventions on the incidence of mental disorders in children and adults, with extended periods of follow-up.