The role of working memory (WM) in children's growth in mathematical problem solving was examined in a longitudinal study of children (N = 127). A battery of tests was administered that assessed problem solving, achievement, WM, and cognitive processing (inhibition, speed, phonological coding) in Grade 1 children, with follow-up testing in Grades 2 and 3. The results were that (a) Grade 1 predictors that contributed unique variance to Grade 3 word problem-solving performance were WM, naming speed, and inhibition and (b) growth in the executive component of WM was significantly related to growth in word problem-solving accuracy. The results support the notion that growth in the executive system of WM is an important predictor of growth in children's problem solving beyond the contribution of cognitive measures of inattention, inhibition, and processing speed as well as achievement measures related to calculation and reading. (Contains 4 tables.)