Objective: Research on monkeys with a forelimb from which sensation is surgically abolished demonstrates that such animals do not use their deafferented limb even though they possess sufficient motor ability to do so, a phenomenon labeled learned nonuse. This dissociation also occurs after neurological injury in humans. Instruments that measure these 2 aspects of motor function are discussed, and the implications of this work for cognitive assessment are explored. Study Design: Literature review. Results and Conclusions: 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 research to independently assess these 2 domains are reliable and valid. Copyright 2005 by the Educational Publishing Foundation.