Background Delayed motor development may occur in children with Down syndrome, cerebral palsy, general developmental delay or children born preterm. It limits the child's exploration of the environment and can hinder cognitive and social‐emotional development. Literature suggests that task‐specific training, such as locomotor treadmill training, facilitates motor development. Objectives To assess the effectiveness of treadmill interventions on locomotor development in children with delayed ambulation or in pre‐ambulatory children (or both), who are under six years of age and who are at risk for neuromotor delay. Search methods In May 2017, we searched CENTRAL, MEDLINE, Embase, six other databases and a number of trials registers. We also searched the reference lists of relevant studies and systematic reviews. Selection criteria We included randomised controlled trials (RCTs) and quasi‐RCTs that evaluated the effect of treadmill intervention in the target population. Data collection and analysis Four authors independently extracted the data. Outcome parameters were structured according to the International Classification of Functioning, Disability and Health model. Main results This is an update of a Cochrane review from 2011, which included five trials. This update includes seven studies on treadmill intervention in 175 children: 104 were allocated to treadmill groups, and 71 were controls. The studies varied in population (children with Down syndrome, cerebral palsy, developmental delay or at moderate risk for neuromotor delay); comparison type (treadmill versus no treadmill; treadmill with versus without orthoses; high‐ versus low‐intensity training); study duration, and assessed outcomes. Due to the diversity of the studies, only data from five studies were used in meta‐analyses for five outcomes: age of independent walking onset, overall gross motor function, gross motor function related to standing and walking, and gait velocity. GRADE assessments of quality of the evidence ranged from high to very low. The effects of treadmill intervention on independent walking onset compared to no treadmill intervention was population dependent, but showed no overall effect (mean difference (MD) ‐2.08, 95% confidence intervals (CI) ‐5.38 to 1.22, 2 studies, 58 children; moderate‐quality evidence): 30 children with Down syndrome benefited from treadmill training (MD ‐4.00, 95% CI ‐6.96 to ‐1.04), but 28 children at moderate risk of developmental delay did not (MD ‐0.60, 95% CI ‐2.34 to 1.14). We found no evidence regarding walking onset in two studies that compared treadmill intervention with and without orthotics in 17 children (MD 0.10, 95% CI ‐5.96 to 6.16), and high‐ versus low‐intensity treadmill interventions in 30 children with Down syndrome (MD ‐2.13, 95% ‐4.96 to 0.70). Treadmill intervention did not improve overall gross motor function (MD 0.88, 95% CI ‐4.54 to 6.30, 2 studies, 36 children; moderate‐quality evidence) or gross motor skills related to standing (MD 5.41, 95% CI ‐1.64 to 12.43, 2 studies, 32 children; low‐quality evidence), and had a negligible improvement in gross motor skills related to walking (MD 4.51, 95% CI 0.29 to 8.73, 2 studies, 32 children; low‐quality evidence). It led to improved walking skills in 20 ambulatory children with developmental delay (MD 7.60, 95% CI 0.88 to 14.32, 1 study) and favourable gross motor skills in 12 children with cerebral palsy (MD 8.00, 95% CI 3.18 to 12.82). A study which compared treadmill intervention with and without orthotics in 17 children with Down syndrome suggested that adding orthotics might hinder overall gross motor progress (MD ‐8.40, 95% CI ‐14.55 to ‐2.25). Overall, treadmill intervention showed a very small increase in walking speed compared to no treadmill intervention (MD 0.23, 95% CI 0.08 to 0.37, 2 studies, 32 children; high‐quality evidence). Treadmill intervention increased walking speed in 20 ambulatory children with developmental delay (MD 0.25, 95% CI 0.08 to 0.42), but not in 12 children with cerebral palsy (MD 0.18, 95% CI ‐0.09 to 0.45). Authors' conclusions This update of the review from 2011 provides additional evidence of the efficacy of treadmill intervention for certain groups of children up to six years of age, but power to find significant results still remains limited. The current findings indicate that treadmill intervention may accelerate the development of independent walking in children with Down syndrome and may accelerate motor skill attainment in children with cerebral palsy and general developmental delay. Future research should first confirm these findings with larger and better designed studies, especially for infants with cerebral palsy and developmental delay. Once efficacy is established, research should examine the optimal dosage of treadmill intervention in these populations.