Groups of children and adolescents of average intelligence, at every age from 6 to 18 years, and college students of slightly above average intelligence were presented with two-disk and three-disk versions of the Tower of Hanoi problem. Criterion was two successive solutions in the minimum number of moves. Ability to reach criterion on the two-disk problem increased to near-perfect performance at age 8. On the three-disk problem, there were two transition periods—one between 7 and 9 years of age, the other between 11 and 14 years of age. The incorrect first move was the largest source of error on the three-disk problem.