Basic principles of magnetic stimulation of biological tissues are reviewed. Noninvasive magnetic stimulation of the brain delivered over sensorimotor areas evokes movements and less commonly paresthesias in contralateral limbs. We have evaluated the maps of motor outputs in patients with (1) congenital mirror movements, which resulted in marked derangement of the map of outputs of distal hand muscles with enlarged and ipsilateral representations; (2) amputations, which resulted in plastic reorganization of motor outputs targeting muscles immediately proximal to the stump; (3) spinal cord injury, which also resulted in enlargement of the map of outputs targeting muscles proximal to the lesion level; and (4) hemispherectomy performed at an early age for intractable seizures, which resulted in the remaining hemisphere controlling ipsilateral arm muscles. These results demonstrate the potential for reorganization in motor systems following lesions in the peripheral as well as in the central nervous system.