After a stroke, recovery that continues beyond 3 or 4 weeks has been attributed to plasticity, a reorganization of the brain in which functions previously performed by the ischemic area are assumed by other ipsilateral or contralateral brain areas. Neuronal plasticity has been variously attributed to redundancy (parallel distributed pathways), changes in synaptic strength, axonal sprouting with formation of new synapses, assumption of function by contralateral homologous cortex, and substitution of uncrossed pathways. Transcranial magnetic stimulation, positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and 128-electrode high-resolution electroencephalography have been successfully applied to demonstrate cortical reorganization after hemiplegia. Recording the motor potential is a promising noninvasive method for the localization of motor control after hemispheric lesions. Most patients with hemiparetic stroke show some improvement, usually during the first 3 to 6 months after the ictus. Improvement and prognosis depend on a number of variables including volume and location of the infarction, age of the patient, and the elimination of risk factors to avoid future episodes (i.e., dietary control of lipids, the elimination of tobacco, and the control of diabetes and hypertension). Currently, emphasis has been placed on fibrinolytic treatment in the first 3 hours to prevent or minimize neurological deficit. Aside from the above listed factors, improvement after stroke may be due to reorganization of the brain, particularly the cerebral cortex, and repair of damaged tissue and recanalization. It is also important to relate such changes to functional improvement and successful rehabilitation.