Background: Unconditional cash transfers (UCTs; provided without obligation) for reducing poverty and vulnerabilities (e.g. orphanhood, old age, or HIV infection) are a social protection intervention addressing a key social determinant of health (income) in low- and middle-income countries (LMICs). The relative effectiveness of UCTs compared with conditional cash transfers (CCTs; provided only if recipients follow prescribed behaviours, e.g. use a health service or attend school) is unknown. Objectives: To assess the effects of UCTs on health services use and health outcomes in children and adults in LMICs. Secondary objectives are to assess the effects of UCTs on social determinants of health and healthcare expenditure, and to compare the effects of UCTs versus CCTs. Search methods: For this update, we searched 15 electronic academic databases, including CENTRAL, MEDLINE and EconLit, in September 2021. We also searched four electronic grey literature databases, websites of key organisations and reference lists of previous systematic reviews, key journals and included study records. Selection criteria: We included both parallel-group and cluster-randomised controlled trials (C-RCTs), quasi-RCTs, cohort studies, controlled before-and-after studies (CBAs), and interrupted time series studies of UCT interventions in children (0 to 17 years) and adults (≥ 18 years) in LMICs. Comparison groups received either no UCT, a smaller UCT or a CCT. Our primary outcomes were any health services use or health outcome. Data collection and analysis: Two review authors independently screened potentially relevant records for inclusion, extracted data and assessed the risk of bias. We obtained missing data from study authors if feasible. For C-RCTs, we generally calculated risk ratios for dichotomous outcomes from crude frequency measures in approximately correct analyses. Meta-analyses applied the inverse variance or Mantel-Haenszel method using a random-effects model. Where meta-analysis was impossible, we synthesised results using vote counting based on effect direction. We assessed the certainty of the evidence using GRADE. Main results: We included 34 studies (25 studies of 20 C-RCTs, six CBAs, and three cohort studies) involving 1,140,385 participants (45,538 children, 1,094,847 adults) and 50,095 households in Africa, the Americas and South-East Asia in our meta-analyses and narrative syntheses. These analysed 29 independent data sets. The 24 UCTs identified, including one basic universal income intervention, were pilot or established government programmes or research experiments. The cash value was equivalent to 1.3% to 81.9% of the annualised gross domestic product per capita. All studies compared a UCT with no UCT; three studies also compared a UCT with a CCT. Most studies carried an overall high risk of bias (i.e. often selection or performance bias, or both). Most studies were funded by national governments or international organisations, or both. Throughout the review, we use the words 'probably' to indicate moderate-certainty evidence, 'may/maybe' for low-certainty evidence, and 'uncertain' for very low-certainty evidence. Health services use. We assumed greater use of any health services to be beneficial. UCTs may not have impacted the likelihood of having used any health service in the previous 1 to 12 months, when participants were followed up between 12 and 24 months into the intervention (risk ratio (RR) 1.04, 95% confidence interval (CI) 1.00 to 1.09; I2 = 2%; 5 C-RCTs, 4972 participants; low-certainty evidence). Health outcomes. At one to two years, UCTs probably led to a clinically meaningful, very large reduction in the likelihood of having had any illness in the previous two weeks to three months (RR 0.79, 95% CI 0.67 to 0.92; I2 = 53%; 6 C-RCTs, 9367 participants; moderate-certainty evidence). UCTs may have increased the likelihood of having been food secure over the previous month, at 13 to 36 months into the intervention (RR 1.25, 95% CI 1.09 to 1.45; I2 = 85%; 5 C-RCTs, 2687 participants; low-certainty evidence). UCTs may have increased participants' level of dietary diversity over the previous week, when assessed with the Household Dietary Diversity Score and followed up 24 months into the intervention (mean difference (MD) 0.59 food categories, 95% CI 0.18 to 1.01; I2 = 79%; 4 C-RCTs, 9347 participants; low-certainty evidence). Despite several studies providing relevant evidence, the effects of UCTs on the likelihood of being moderately stunted and on the level of depression remain uncertain. We found no study on the effect of UCTs on mortality risk. Social determinants of health. UCTs probably led to a clinically meaningful, moderate increase in the likelihood of currently attending school, when assessed at 12 to 24 months into the intervention (RR 1.06, 95% CI 1.04 to 1.09; I2 = 0%; 8 C-RCTs, 7136 participants; moderate-certainty evidence). UCTs may have reduced the likelihood of households being extremely poor, at 12 to 36 months into the intervention (RR 0.92, 95% CI 0.87 to 0.97; I2 = 63%; 6 C-RCTs, 3805 participants; low-certainty evidence). The evidence was uncertain for whether UCTs impacted livestock ownership, participation in labour, and parenting quality. Healthcare expenditure. Evidence from eight cluster-RCTs on healthcare expenditure was too inconsistent to be combined in a meta-analysis, but it suggested that UCTs may have increased the amount of money spent on health care at 7 to 36 months into the intervention (low-certainty evidence). Equity, harms and comparison with CCTs. The effects of UCTs on health equity (or unfair and remedial health inequalities) were very uncertain. We did not identify any harms from UCTs. Three cluster-RCTs compared UCTs versus CCTs with regard to the likelihood of having used any health services or had any illness, or the level of dietary diversity, but evidence was limited to one study per outcome and was very uncertain for all three. Authors' conclusions: This body of evidence suggests that unconditional cash transfers (UCTs) may not impact a summary measure of health service use in children and adults in LMICs. However, UCTs probably or may improve some health outcomes (i.e. the likelihood of having had any illness, the likelihood of having been food secure, and the level of dietary diversity), two social determinants of health (i.e. the likelihoods of attending school and being extremely poor), and healthcare expenditure. The evidence on the relative effectiveness of UCTs and CCTs remains very uncertain.