Stem cells in the kidney. The kidney is derived from the ureteric bud and the metanephrogenic mesenchyme, and these two progenitor cells differentiate into more than 26 different cell types in the adult kidney. The ureteric bud contains the precursor of the epithelial cells of the collecting duct and the renal mesenchyme contains precursors of all the epithelia of the rest of the nephron, endothelial cell precursors and stroma cells, but the relatedness among these cells is unclear. A single metanephric mesenchymal cell can generate all the epithelial cells of the nephron (except the collecting duct), indicating that the kidney contains epithelial stem cells. It is currently unknown whether these stem cells also are present in the adult kidney but experience in other organs makes this likely. It also is unclear whether embryonic renal epithelial stem cells can generate other cell types, but preliminary studies in our laboratory suggest that they can differentiate into myofibroblasts, smooth muscle, and perhaps endothelial cells, indicating that they are pluripotent renal stem cells. The important problem to be solved now is the identification and location of adult renal stem cells. This article discusses work done in other organs and in renal development that we believe may be useful for the resolution of this problem.