The use of stem cells in regenerative strategies has been studied for many other organ systems and cell types, including skin, eye, cartilage, bone, kidney, lung, endometrium, vascular endothelium, smooth muscle, and striated muscle, among others. In fact, the potential for stem cell regeneration of damaged organs and tissues is virtually limitless. However, there are numerous obstacles to be overcome before stem cell therapies can become a widespread clinical reality. Only hematopoietic stem cells have been adequately characterized by surface markers so that they can be unambiguously identified, a prerequisite for reliable clinical applications. The pathways for differentiating stem cells into specific cellular phenotypes are largely unknown, and there is little ability at present to control the migration of transplanted cells or predict the response of the cells to the environment of diseased organs. Some strategies may employ the coadministration of scaffolding, artificial extracellular matrix, and/or growth factors to orchestrate differentiation of stem cells and their organization into appropriate constituents of the organ. There is currently no way to image stem cells in vivo after transplantation into humans, and it will be necessary to develop techniques to accomplish this. Fortunately, stem cells can be engineered before transplantation to contain a contrast agent that may make this feasible. The potential for tumor formation and the problems associated with immune rejection are impediments, and it will also be necessary to develop techniques for ensuring vascularization of regenerated tissues. There already are many strategies for cell replacement, including vasoactive endothelial growth factor (VEGF) coadministration to foster vascularization of the transplant. Some strategies also include genetically engineering stem cells to have an inducible suicide gene so that the cells can be easily eradicated in the event of tumor formation or another complication. The potential for stem cell therapies to revolutionize medical care is extraordinary, and disorders such as myocardial infarction, diabetes, and Parkinson's disease, among many others, will become potentially curable. However, such stem cell–based therapies are still at a very early stage of development, and perfection of techniques for clinical transplantation of predictable, well-characterized cells is going to be a difficult and lengthy undertaking.