Basic behavioral neuroscience research with monkeys has given rise to an efficacious new approach to the rehabilitation of movement after stroke, cerebral palsy, traumatic brain injury, and other types of neurological injury in humans termed Constraint-Induced Movement therapy or CI therapy. For the upper extremity, the treatment involves intensive training of the more affected arm by "shaping," the application of a number of other behavioral techniques, and prolonged constraint of use of the less affected arm. CI therapy has been shown to produce large changes in the organization and function of the brain. This result points to the fact that behavior can have a profound effect on the nervous system that is greater than is generally recognized, and harnessing this brain plasticity by behavioral means has promise for the development of new treatments in the field of rehabilitation.