We investigated the effect of cognitive training of executive functions on children's cognitive outcomes. To address this issue, a systematic meta-analysis of published research articles on cognitive training interventions was performed considering children's age, training duration, -procedure, and -technology in moderator analyses. The results (N = 57) of a random-effects-model showed that cognitive training was effective with a total effect size of g = 23. The training was more effective for younger compared to older children. Training benefits were found for near- and far-transfer tasks. The largest gains for a near-transfer skill were found for working memory. Both numeracy and literacy skills profited from training. Computer training was very effective, however, only a few studies used mobile technology. Non-adaptive training was associated with greater effect sizes and both group and individual training were similarly effective. Verbal feedback was important for younger children. School was an effective context for training, however, only a few studies were conducted at home or at the lab. The findings are discussed and advocate an early start of cognitive training interventions.