This study explored the contribution of the phonological and executive working memory (WM) systems to 205 (102 girls, 103 boys, 6 to 9 years old) elementary school children's fluid and crystallized intelligence. The results show that (a) a 3-factor structure (phonological short-term memory [STM], visual-spatial WM, and verbal WM) was comparable between age groups, (b) controlled attention and STM storage accounted for 67% of the age-related variance in WM, (c) effect sizes for direct paths from WM were substantially larger when predicting fluid intelligence than crystallized intelligence, and (d) the contribution of STM to intelligence was isolated to reading. The results suggest that the development of WM is distinct from STM, controlled attention plus storage accounted for age-related WM changes, and WM underlies age-related changes in both fluid and crystallized intelligence. (Contains 6 footnotes, 6 tables, and 4 figures.)