BACKGROUND: Paediatric flat feet are a common presentation in primary care; reported prevalence approximates 15%. A minority of flat feet can hurt and limit gait. There is no optimal strategy, nor consensus, for using foot orthoses (FOs) to treat paediatric flat feet. OBJECTIVES: To assess the benefits and harms of foot orthoses for treating paediatric flat feet. SEARCH METHODS: We searched CENTRAL, MEDLINE, and Embase to 01 September 2021, and two clinical trials registers on 07 August 2020. SELECTION CRITERIA: We identified all randomised controlled trials (RCTs) of FOs as an intervention for paediatric flat feet. The outcomes included in this review were pain, function, quality of life, treatment success, and adverse events. Intended comparisons were: any FOs versus sham, any FOs versus shoes, customised FOs (CFOs) versus prefabricated FOs (PFOs). DATA COLLECTION AND ANALYSIS: We followed standard methods recommended by Cochrane. MAIN RESULTS: We included 16 trials with 1058 children, aged 11 months to 19 years, with flexible flat feet. Distinct flat foot presentations included asymptomatic, juvenile idiopathic arthritis (JIA), symptomatic and developmental co-ordination disorder (DCD). The trial interventions were FOs, footwear, foot and rehabilitative exercises, and neuromuscular electrical stimulation (NMES). Due to heterogeneity, we did not pool the data. Most trials had potential for selection, performance, detection, and selective reporting bias. No trial blinded participants. We present the results separately for asymptomatic (healthy children) and symptomatic (children with JIA) flat feet. The certainty of evidence was very low to low, downgraded for bias, imprecision, and indirectness. Three comparisons were evaluated across trials: CFO versus shoes; PFO versus shoes; CFO versus PFO. Asymptomatic flat feet 1. CFOs versus shoes (1 trial, 106 participants): low-quality evidence showed that CFOs result in little or no difference in the proportion without pain (10-point visual analogue scale (VAS)) at one year (risk ratio (RR) 0.85, 95% confidence interval (CI) 0.67 to 1.07); absolute decrease (11.8%, 95% CI 4.7% fewer to 15.8% more); or on withdrawals due to adverse events (RR 1.05, 95% CI 0.94 to 1.19); absolute effect (3.4% more, 95% CI 4.1% fewer to 13.1% more). 2. PFOs versus shoes (1 trial, 106 participants): low to very-low quality evidence showed that PFOs result in little or no difference in the proportion without pain (10-point VAS) at one year (RR 0.94, 95% CI 0.76 to 1.16); absolute effect (4.7% fewer, 95% CI 18.9% fewer to 12.6% more); or on withdrawals due to adverse events (RR 0.99, 95% CI 0.79 to 1.23). 3. CFOs versus PFOs (1 trial, 108 participants): low-quality evidence found no difference in the proportion without pain at one year (RR 0.93, 95% CI 0.73 to 1.18); absolute effect (7.4% fewer, 95% CI 22.2% fewer to 11.1% more); or on withdrawal due to adverse events (RR 1.00, 95% CI 0.90 to 1.12). Function and quality of life (QoL) were not assessed. Symptomatic (JIA) flat feet 1. CFOs versus shoes (1 trial, 28 participants, 3-month follow-up): very low-quality evidence showed little or no difference in pain (0 to 10 scale, 0 no pain) between groups (MD -1.5, 95% CI -2.78 to -0.22). Low-quality evidence showed improvements in function with CFOs (Foot Function Index - FFI disability, 0 to 100, 0 best function; MD -18.55, 95% CI -34.42 to -2.68), child-rated QoL (PedsQL, 0 to 100, 100 best quality; MD 12.1, 95% CI -1.6 to 25.8) and parent-rated QoL (PedsQL MD 9, 95% CI -4.1 to 22.1) and little or no difference between groups in treatment success (timed walking; MD -1.33 seconds, 95% CI -2.77 to 0.11), or withdrawals due to adverse events (RR 0.58, 95% CI 0.11 to 2.94); absolute difference (9.7% fewer, 20.5 % fewer to 44.8% more). 2. PFOs versus shoes (1 trial, 25 participants, 3-month follow-up): very low-quality evidence showed little or no difference in pain between groups (MD 0.02, 95% CI -1.94 to 1.98). Low-quality evidence showed no difference between groups in function (FFI-disability MD -4.17, 95% CI -24.4 to 16.06), child-rated QoL (PedsQL MD -3.84, 95% CI -19 to 11.33), or parent-rated QoL (PedsQL MD -0.64, 95% CI -13.22 to 11.94). 3. CFOs versus PFOs (2 trials, 87 participants): low-quality evidence showed little or no difference between groups in pain (0 to 10 scale, 0 no pain) at 3 months (MD -1.48, 95% CI -3.23 to 0.26), function (FFI-disability MD -7.28, 95% CI -15.47 to 0.92), child-rated QoL (PedsQL MD 8.6, 95% CI -3.9 to 21.2), or parent-rated QoL (PedsQL MD 2.9, 95% CI -11 to 16.8). AUTHORS' CONCLUSIONS: Low to very low-certainty evidence shows that the effect of CFOs (high cost) or PFOs (low cost) versus shoes, and CFOs versus PFOs on pain, function and HRQoL is uncertain. This is pertinent for clinical practice, given the economic disparity between CFOs and PFOs. FOs may improve pain and function, versus shoes in children with JIA, with minimal delineation between costly CFOs and generic PFOs. This review updates that from 2010, confirming that in the absence of pain, the use of high-cost CFOs for healthy children with flexible flat feet has no supporting evidence, and draws very limited conclusions about FOs for treating paediatric flat feet. The availability of normative and prospective foot development data, dismisses most flat foot concerns, and negates continued attention to this topic. Attention should be re-directed to relevant paediatric foot conditions, which cause pain, limit function, or reduce quality of life. The agenda for researching asymptomatic flat feet in healthy children must be relegated to history, and replaced by a targeted research rationale, addressing children with indisputable foot pathology from discrete diagnoses, namely JIA, cerebral palsy, congenital talipes equino varus, trisomy 21 and Charcot Marie Tooth. Whether research resources should continue to be wasted on studying flat feet in healthy children that do not hurt, is questionable. Future updates of this review will address only relevant paediatric foot conditions.