The ubiquitous power-law of practice has been a touch-tone of cognitive models. It predicts that the speed of performance of a task will improve as the power of the number of times that the task is performed. In this paper we derive the power-law from a graph dynamical theory of learning by considering change^ in problem->space graph topology due to the addition of operators,and alterations in the decision-procedure used to decide which operator to apply at a particular.The general approach of applying dynamical principle.-^ to cognitive problems holds much promise in unifying other' areas of learning and intelligent activity.