Cell therapy is the process of using intact cells to treat a disease or other unwanted condition. For genetic disorders, cell therapy is based on the premise that allogeneic, genetically normal, stem or progenitor cells may be used to repopulate the cells of a genetically defective tissue, effectively correcting the underlying genetic disorder. The clinical potential of cell therapy is enormous. To begin to appreciate the possible uses of cell therapy in medicine, it is important to understand the relationship of cell therapy to other treatment strategies, such as drugs and gene therapy. First, it is unlikely that a single treatment strategy will be best in all clinical situations. Thus, we prefer to consider cell therapy as a complementary approach to the treatment of disease rather than a competing avenue of investigation. Second, pharmaceuticals, which are currently the foundation of the practice of medicine, will generally exert an effect by altering cell function, supplying a missing cell product, or eliminating unwanted cells. Drug therapy cannot induce the regeneration of genetically normal tissue if the underlying genetic mutation, which exists in all cells, remains unaltered. Finally, gene therapy, which is also a potentially very important approach to correcting genetic disorders, requires some capacity to insert a gene into a cell and achieve long-term expression with appropriate regulation. In many cases this presents a difficult challenge to investigators. Hence, cell therapy, in which all regulatory elements, even those that are unknown, can operate properly, may be more readily developed for clinical applications. However, as mentioned above, these two strategies are complementary and both must be explored to determine the best approach for any given disorder. The development of novel therapeutic strategies is a great challenge. It is tempting to rest on established treatments, or pursue treatments with little risk; however, this approach will usually not lead to true cures for many debilitating diseases. Although diabetes mellitus is not a ‘classic’ genetic disease, it offers an instructive example of the value of developing cell therapy. Exogenous insulin administered twice daily is an essential treatment for individuals with diabetes mellitus; however, long-term complications persist, and affected individuals often have many related health problems. Investigators today recognize the importance of insulin therapy, but nonetheless vigorously pursue pancreatic islet cell transplantation as a cell-therapeutic strategy with the potential to truly cure these patients. Cell therapy soon after diagnosis may result in physiologic insulin regulation and eliminate associated problems of diabetes.