BACKGROUND: Insufficient physical activity is one of four primary risk factors for non-communicable diseases such as stroke, heart disease, type 2 diabetes, cancer and chronic lung disease. As few as one in five children aged 5 to 17 years have the physical activity recommended for health benefits. The outside-school hours period contributes around 30% of children's daily physical activity and presents a key opportunity for children to increase their physical activity. Testing the effects of interventions in outside-school hours childcare settings is required to assess the potential to increase physical activity and reduce disease burden. OBJECTIVES: To assess the effectiveness, cost-effectiveness and associated adverse events of interventions designed to increase physical activity in children aged 4 to 12 years in outside-school hours childcare settings. SEARCH METHODS: We searched CENTRAL, MEDLINE, Embase, ERIC and SportsDISCUS to identify eligible trials on 18 August 2020. We searched two databases, three trial registries, reference lists of included trials and handsearched two physical activity journals in August 2020. We contacted first and senior authors on articles identified for inclusion for ongoing or unpublished potentially relevant trials in August 2020. SELECTION CRITERIA: We included randomised controlled trials, including cluster-randomised controlled trials, of any intervention primarily aimed at increasing physical activity in children aged 4 to 12 years in outside-school hours childcare settings compared to usual care. To be eligible, the interventions must have been delivered in the context of an existing outside-school hours childcare setting (i.e. childcare that was available consistently throughout the school week/year), and not set up in the after-school period for the purpose of research. Two review authors independently screened titles and abstracts of identified papers with discrepancies resolved via a consensus discussion. A third review author was not required to resolve disagreements. DATA COLLECTION AND ANALYSIS: Two review authors independently extracted data and assessed the risk of bias of included trials with discrepancies resolved via a consensus discussion; a third review author was not required to resolve disagreements. For continuous measures of physical activity, we reported the mean difference (MD) with 95% confidence intervals (CIs) in random-effects models using the generic inverse variance method for each outcome. For continuous measures, when studies used different scales to measure the same outcome, we used standardised mean differences (SMDs). We conducted assessments of risk of bias of all outcomes and evaluated the certainty of evidence (GRADE approach) using standard Cochrane procedures. MAIN RESULTS: We included nine trials with 4458 participants. Five trials examined the effectiveness of staff-based interventions to change practice in the outside-school hours childcare setting (e.g. change in programming, activities offered by staff, staff facilitation/training). Two trials examined the effectiveness of staff- and parent-based interventions (e.g. parent newsletters/telephone calls/messages or parent tool-kits in addition to staff-based interventions), one trial assessed staff- and child-based intervention (e.g. children had home activities to emphasise physical activity education learnt during outside-school hours childcare sessions in addition to staff-based interventions) and one trial assessed child-only based intervention (i.e. only children were targeted).  We judged two trials as free from high risk of bias across all domains. Of those studies at high risk of bias, it was across domains of randomisation process, missing outcome data and measurement of the outcome. There was low-certainty evidence that physical activity interventions may have little to no effect on total daily moderate-to-vigorous physical activity compared to no intervention (MD 1.7 minutes, 95% CI -0.42 to 3.82; P = 0.12; 6 trials; 3042 children). We were unable to pool data on proportion of the OSHC session spent in moderate-to-vigorous physical activity in a meta-analysis. Both trials showed an increase in proportion of session spent in moderate-to-vigorous physical activity (moderate-certainty evidence) from 4% to 7.3% of session time; however, only one trial was statistically significant. There was low-certainty evidence that physical activity interventions may lead to little to no reduction in body mass index (BMI) as a measure of cardiovascular health, compared to no intervention (SMD -0.17, 95% CI -0.44 to 0.10; P = 0.22; 4 trials, 1684 children). Physical activity interventions that were delivered online were more cost-effective than in person. Combined results suggest that staff-and-parent and staff-and-child-based interventions may lead to a small increase in overall daily physical activity and a small reduction or no difference in BMI. Process evaluation was assessed differently by four of the included studies, with two studies reporting improvements in physical activity practices, one reporting high programme satisfaction and one high programme fidelity. The certainty of the evidence for these outcomes was low to moderate. Finally, there was very low-certainty evidence that physical activity interventions in outside-school hours childcare settings may increase cardiovascular fitness. No trials reported on quality of life or adverse outcomes. Trials reported funding from local government health grants or charitable funds; no trials reported industry funding. AUTHORS' CONCLUSIONS: Although the review included nine trials, the evidence for how to increase children's physical activity in outside-school hours care settings remains limited, both in terms of certainty of evidence and magnitude of the effect. Of the types of interventions identified, when assessed using GRADE there was low-certainty evidence that multi-component interventions, with a specific physical activity goal may have a small increase in daily moderate-to-vigorous physical activity and a slight reduction in BMI. There was very low-certainty evidence that interventions increase cardiovascular fitness. By contrast there was moderate-certainty evidence that interventions were effective for increasing proportion of time spent in moderate-to-vigorous physical activity, and online training is cost-effective.