Stem cells are characterized by low cycle time, which has allowed us to identify such cells in the mature kidney. These putative stem cells are located mostly outside the renal tubule and are concentrated in the papilla of the kidney potentially under the urinary epithelium of the papilla. Clonal analysis of these cells shows that they can differentiate into epithelial, neuronal, and other uncharacterized cells. Induction of ischemic renal failure resulted in increased proliferation of these papillary cells. Injection of these cells under the renal capsule led to their incorporation into various tubule segments. It is likely that these stem cells sense a “damage” signal from the cortex resulting in proliferation followed by migration to the site of injury.