A growing trend in developmental psycholinguistics is to relate linguistic development to the development of other cognitive systems. Jakubowicz (2005, 2011) in particular argued that the processing of a complex sentence requires considerable working memory (WM) resources and that these resources are limited in young children, which would explain their non-adult grammar. The present research aims to clarify the relationship between WM and complex syntax, in comprehension, repetition, and spontaneous production, in 48 typically-developing children aged 5 to 12. The results demonstrate a strong age effect for all measures of WM and syntax. They also reveal strong correlations between scores on simple and complex spans and syntactic performance. Finally, it shows the highly predictive value of WM capacities on the acquisition of syntactic skills in both comprehension and production. In particular, the complex-span task, measuring counting span, explains the largest part of the variance in the spontaneous production of embedded clauses.