BACKGROUND: Objectives were to determine whether the use of ultrasound guidance offers any clinical advantage in the performance of neuraxial or peripheral nerve blocks in children in terms of increasing the success rate or decreasing the rate of complications. METHODS: We searched the following databases to March 2015: Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE (OvidSP), EMBASE (OvidSP), and Scopus (to January 2015). We included all parallel randomized controlled trials that evaluated the effect of ultrasound guidance to perform a regional blockade technique in children. We assessed the selected studies for risk of bias with the Cochrane collaboration tool. We graded the level of evidence for each outcome according to the Grading of Recommendations Assessment, Development and Evaluation (GRADE) working group scale. RESULTS: We included 20 studies (1241 participants). Ultrasound guidance increases the success rate by decreasing the occurrence of a failed block: risk difference -0.11 (95% confidence interval [CI], -0.17 to -0.05); I 64%; number needed for additional beneficial outcome for peripheral nerve blocks 6 (95% CI, 5-8). The younger the child, the greater the benefit. From 8 studies (414 participants), pain scores at 1 hour in the postanesthesia care unit were reduced when ultrasound guidance was used; however, the clinical relevance of the difference was unclear (equivalent to -0.2 on a scale from 0 to 10). From 8 studies (358 participants), block duration was longer when ultrasound guidance was used: standardized mean difference (SMD) 1.21 (95% CI, 0.76-1.65: I 73%; equivalent to 62 minutes). Here again, younger children benefited the most from ultrasound guidance. Time to perform the procedure was reduced when ultrasound guidance was used for prescanning before a neuraxial block (SMD -1.97; 95% CI, -2.41 to -1.54; I 0%; equivalent to 2.4 minutes) or as an out-of-plane technique (SMD -0.68; 95% CI, -0.96 to -0.40; I 0%; equivalent to 94 seconds). From 2 studies (122 participants), ultrasound guidance reduced the number of needle passes required to perform the block: SMD -0.90 (95% CI, -1.27 to -0.52; I 0%; equivalent to 0.6 needle pass per participant). From 2 studies (204 participants), we could not demonstrate a difference in the incidence of bloody puncture when ultrasound guidance was used for neuraxial blockade, but we found that the number of participants was well below the optimal information size (risk difference -0.07, 95% CI, -0.19 to 0.04). No major complications were reported for any of the 1241 participants. We rated the quality of evidence as high for success, pain scores at 1 hour, block duration, time to perform the block, and number of needle passes. We rated the quality of evidence as low for bloody punctures. CONCLUSIONS: Ultrasound guidance seems advantageous, particularly in young children, for whom it improves the success rate and increases block duration. Additional data are required before conclusions can be drawn on the effect of ultrasound guidance in reducing the rate of bloody puncture.