Fifteen years ago, a study showed that the brains of London cab drivers had an enlargement in the hippocampus, a brain area associated with navigation. But questions remained: Did the experience of navigating London's complex system of streets change their brains, or did only the people with larger hippocampi succeed in becoming cab drivers? Now, scientists have determined that learning detailed navigation information causes the hippocampal brain changes. The findings establish a critical link between structural and functional brain alteration.