Technology-based dietary assessment has promising benefits on improving accuracy and reducing cost of dietary data collection. The validity of technology-based tools in children/adolescents was examined. A systematic literature search was performed to identify studies published till September 2019. In total, 26 studies with 29 validation-related estimations were selected; 13 web-based 24-hdietary recalls (image assisted; n = 12, drop-down food list; n = 13), 4 mobile applications and 3 web-based dietary records (image based; n = 2 and image assisted; n = 1, drop-down food list; n = 3) and 6 web-based food frequency questionnaires (FFQs) (image assisted; n = 3). Fourteen studies were addressed to adolescents, ten studies to children (principally 9–11 years old) and two studies to mixed study samples. Validation was mostly performed through 24-h dietary recalls and dietary records while in some cases objective methods were used. Image-assisted 24-h dietary recalls presented good level of agreement with paper-based methods, yet principally in adolescents. Mobile-application dietary records with image-based facilities and drop-down food lists were found to accurately record dietary intake even when compared with objective methods. FFQs, mainly examined in adolescents, had good level of agreement with the “gold standard” dietary records method. The validity of technology-based methods could be supported yet various technical/methodological issues need better clarification.