In the adult kidney, different populations of progenitor cells (or stem cells) have been identified. These cells may represent a remnant of embryonic stem cells in the adult tissue, or populations of bone-marrow-derived stem cells homed within the kidney and modified by the local microenvironment. This modification may be the expression of a partial commitment or of different degrees of maturation. Resident stem cells may account for the growth of the organ during development, for the physiological cell turnover, and for the repair of kidney damage. In addition, stem cells derived from the bone marrow and migrated through the circulation to the site of the damage may contribute to tissue repair. Preliminary studies suggest that this regenerative potential of stem cells could be exploited for therapeutic purposes by administration of ex vivo expanded stem cell populations or by development of strategies aimed to expand and differentiate local stem cells.