BACKGROUND: Community-based primary-level workers (PWs) are an important strategy for addressing gaps in mental health service delivery in low- and middle-income countries.  OBJECTIVES: To evaluate the effectiveness of PW-led treatments for persons with mental health symptoms in LMICs, compared to usual care.  SEARCH METHODS: MEDLINE, Embase, CENTRAL, ClinicalTrials.gov, ICTRP, reference lists (to 20 June 2019).   SELECTION CRITERIA: Randomised trials of PW-led or collaborative-care interventions treating people with mental health symptoms or their carers in LMICs.  PWs included: primary health professionals (PHPs), lay health workers (LHWs), community non-health professionals (CPs).  DATA COLLECTION AND ANALYSIS: Seven conditions were identified apriori and analysed by disorder and PW examining recovery, prevalence, symptom change, quality-of-life (QOL), functioning, service use (SU), and adverse events (AEs).  Risk ratios (RRs) were used for dichotomous outcomes; mean difference (MDs), standardised mean differences (SMDs), or mean change differences (MCDs) for continuous outcomes.  For SMDs, 0.20 to 0.49 represented small, 0.50 to 0.79 moderate, and ≥0.80 large clinical effects.  Analysis timepoints: T1 (<1 month), T2 (1-6 months), T3 ( >6 months) post-intervention.  MAIN RESULTS: Description of studies 95 trials (72 new since 2013) from 30 LMICs (25 trials from 13 LICs).  Risk of bias Most common: detection bias, attrition bias (efficacy), insufficient protection against contamination.  Intervention effects *Unless indicated, comparisons were usual care at T2.  "Probably", "may", or "uncertain" indicates "moderate", "low," or "very low" certainty evidence.   Adults with common mental disorders (CMDs) LHW-led interventions a. may increase recovery (2 trials, 308 participants; RR 1.29, 95%CI 1.06 to 1.56); b. may reduce prevalence (2 trials, 479 participants; RR 0.42, 95%CI 0.18 to 0.96); c. may reduce symptoms (4 trials, 798 participants; SMD -0.59, 95%CI -1.01 to -0.16); d. may improve QOL (1 trial, 521 participants; SMD 0.51, 95%CI 0.34 to 0.69); e. may slightly reduce functional impairment (3 trials, 1399 participants; SMD -0.47, 95%CI -0.8 to -0.15); f. may reduce AEs (risk of suicide ideation/attempts); g. may have uncertain effects on SU. Collaborative-care a. may increase recovery (5 trials, 804 participants; RR 2.26, 95%CI 1.50 to 3.43); b. may reduce prevalence although the actual effect range indicates it may have little-or-no effect (2 trials, 2820 participants; RR 0.57, 95%CI 0.32 to 1.01); c. may slightly reduce symptoms (6 trials, 4419 participants; SMD -0.35, 95%CI -0.63 to -0.08); d. may slightly improve QOL (6 trials, 2199 participants; SMD 0.34, 95%CI 0.16 to 0.53); e. probably has little-to-no effect on functional impairment (5 trials, 4216 participants; SMD -0.13, 95%CI -0.28 to 0.03); f. may reduce SU (referral to MH specialists);  g. may have uncertain effects on AEs (death). Women with perinatal depression (PND) LHW-led interventions a. may increase recovery (4 trials, 1243 participants; RR 1.29, 95%CI 1.08 to 1.54); b. probably slightly reduce symptoms (5 trials, 1989 participants; SMD -0.26, 95%CI -0.37 to -0.14); c. may slightly reduce functional impairment (4 trials, 1856 participants; SMD -0.23, 95%CI -0.41 to -0.04); d. may have little-to-no effect on AEs (death);  e. may have uncertain effects on SU. Collaborative-care a. has uncertain effects on symptoms/QOL/SU/AEs. Adults with post-traumatic stress (PTS) or CMDs in humanitarian settings LHW-led interventions a. may slightly reduce depression symptoms (5 trials, 1986 participants; SMD -0.36, 95%CI -0.56 to -0.15); b. probably slightly improve QOL (4 trials, 1918 participants; SMD -0.27, 95%CI -0.39 to -0.15); c. may have uncertain effects on symptoms (PTS)/functioning/SU/AEs. PHP-led interventions a. may reduce PTS symptom prevalence (1 trial, 313 participants; RR 5.50, 95%CI 2.50 to 12.10) and depression prevalence (1 trial, 313 participants; RR 4.60, 95%CI 2.10 to 10.08);  b. may have uncertain effects on symptoms/functioning/SU/AEs.   Adults with harmful/hazardous alcohol or substance use LHW-led interventions a. may increase recovery from harmful/hazardous alcohol use although the actual effect range indicates it may have little-or-no effect (4 trials, 872 participants; RR 1.28, 95%CI 0.94 to 1.74); b. may have little-to-no effect on the prevalence of methamphetamine use (1 trial, 882 participants; RR 1.01, 95%CI 0.91 to 1.13) and  functional impairment (2 trials, 498 participants; SMD -0.14, 95%CI -0.32 to 0.03); c. probably slightly reduce risk of harmful/hazardous alcohol use (3 trials, 667 participants; SMD -0.22, 95%CI -0.32 to -0.11);  d. may have uncertain effects on SU/AEs. PHP/CP-led interventions a. probably have little-to-no effect on recovery from harmful/hazardous alcohol use (3 trials, 1075 participants; RR 0.93, 95%CI 0.77 to 1.12) or QOL (1 trial, 560 participants; MD 0.00, 95%CI -0.10 to 0.10); b. probably slightly reduce risk of harmful/hazardous alcohol and substance use (2 trials, 705 participants; SMD -0.20, 95%CI -0.35 to -0.05; moderate-certainty evidence); c. may have uncertain effects on prevalence (cannabis use)/SU/AEs. PW-led interventions for alcohol/substance dependence a. may have uncertain effects.  Adults with severe mental disorders *Comparisons were specialist-led care at T1. LHW-led interventions a. may have little-to-no effect on caregiver burden (1 trial, 253 participants; MD -0.04, 95%CI -0.18 to 0.11);  b. may have uncertain effects on symptoms/functioning/SU/AEs.  PHP-led or collaborative-care a. may reduce functional impairment (7 trials, 874 participants; SMD -1.13, 95%CI -1.78 to -0.47); b. may have uncertain effects on recovery/relapse/symptoms/QOL/SU.  Adults with dementia and carers PHP/LHW-led carer interventions a. may have little-to-no effect on the severity of behavioural symptoms in dementia patients (2 trials, 134 participants; SMD -0.26, 95%CI -0.60 to 0.08); b. may reduce carers' mental distress (2 trials, 134 participants; SMD -0.47, 95%CI -0.82 to -0.13);  c. may have uncertain effects on QOL/functioning/SU/AEs. Children with PTS or CMDs LHW-led interventions a. may have little-to-no effect on PTS symptoms (3 trials, 1090 participants; MCD -1.34, 95%CI -2.83 to 0.14); b. probably have little-to-no effect on depression symptoms (3 trials, 1092 participants; MCD -0.61, 95%CI -1.23 to 0.02) or on functional impairment (3 trials, 1092 participants; MCD -0.81, 95%CI -1.48 to -0.13);  c. may have little-or-no effect on AEs. CP-led interventions a. may have little-to-no effect on depression symptoms (2 trials, 602 participants; SMD -0.19, 95%CI -0.57 to 0.19) or on AEs;  b. may have uncertain effects on recovery/symptoms(PTS)/functioning. AUTHORS' CONCLUSIONS: PW-led interventions show promising benefits in improving outcomes for CMDs, PND, PTS, harmful alcohol/substance use, and dementia carers in LMICs.