Research on monkeys with a single forelimb from which sensation is surgically abolished demonstrates that such animals do not use their deafferented limb even though they possess sufficient motor innervation to do so, a phenomenon labeled learned nonuse. This dissociation also occurs after neurological injury in humans. Instruments that measure these two aspects of motor function are discussed. The effects of a neurological injury may differ widely in regard to motor ability assessed on a laboratory performance test in which movements are requested and actual spontaneous use of an extremity in real-world settings, indicating that these parameters need to be evaluated separately. The methods used in Constraint-Induced Movement therapy (CI therapy) research to independently assess these two domains are reliable and valid. We suggest that these tests have applicability beyond studies involving CI therapy for stroke and may be of value for determining motor status in other types of motor disorders and with other types of treatment. The learned nonuse formulation also predicts that a rehabilitation treatment may have differential effects on motor performance made on request and actual spontaneous amount of use of a more affected upper extremity in the life situation. CI therapy produces improvements in the former, but focuses attention on the latter and, in fact, spontaneous use of the limb is where this intervention has by far its greatest effect. The evidence suggests that this result is driven by use of a "transfer package" of techniques, which can be used with other therapies to increase the transfer of improvements made in the clinic to the life situation. The use of CI therapy in humans began with the upper extremity after stroke and was then extended for the upper extremity to cerebral palsy in young children (8 months to 8 years old) and traumatic brain injury. A form of CI therapy was developed for the lower extremities and was used effectively after stroke, spinal cord injury, and fractured hip. Adaptations of CI therapy have also been developed for aphasia (CI aphasia therapy), focal hand dystonia in musicians and phantom limb pain. The range of these applications suggests that CI therapy is not only a treatment for stroke, for which it is most commonly used, but for learned nonuse in general, which manifests as excess motor disability in a number of conditions which until now have been refractory to treatment.