We previously believed that brain disorders could not be treated. However, brain-imaging techniques have demonstrated functional localization and the recovery of damaged areas of the brain. Through the use of various radiopharmaceuticals, positron emission tomography (PET) allows in vivo imaging of regional cerebral functions, including cerebral blood flow, molecular metabolism, and receptor binding capacity. In addition, PET demonstrates neural plasticity and compensation for brain damage. This paper discusses the plasticity and compensation of the brain revealed by our PET studies.