As children grow up, they switch from procedure-based to memory-based strategies for problem solving. However, the changes in brain circuitry underlying this switch are unknown. In a functional MRI analysis of 7–9-year-old children who were asked to solve simple addition problems (such as 3 + 5 =?), the authors found that the switch to memory-based strategies involved an increase in hippocampal activity, a decrease in prefrontal–parietal activity and increased hippocampus–neocortex connectivity. The efficiency of the memory-based strategy continued to improve through adolescence and into adulthood. This finding highlights the importance of functional reorganization in the hippocampus and neocortex during this developmental period.