Human embryonic stem (hES) cell technology has advanced for more than a decade and has come to be regarded as "the gold standard" in the stem cell fi eld.1,2 Th ese cells have met the most stringent standards for pluripotency and demonstrate immense promise for diagnostic and therapeutic capabilities.3 Nevertheless, hES research has given rise to social confl ict and nations remain divided regarding its regulation. 4 We aim to distinguish between the scientifi c and clinical realities of hES technology and highlight the ethical, legal, and social issues concerning its use in the United States (US). In light of obstacles that impede the progression of these technologies, we consider what the future may look like for stem cell research. Scientifi c and Clinical Applications of hES CellsSince isolation in 1981, nonhuman embryonic stem (ES) cells have proven suffi cient to give rise to all tissues of the adult organism across all three embryological germ layers.5-8 Twenty-seven years later in 1998, the fi rst hES cells were isolated from frozen embryos and have since contributed enormously to our understanding of cellular pluripotency. 3 , 9 Although no hES treatments are available for clinical use in nonstudy settings in the US, the fi rst FDA-approved Phase I clinical trials for hES technology are underway assessing the safety of hES cell-derived oligodendrocyte progenitor cells in patients with spinal cord injury and retinal pigmented epithelial cells in patients with age-related macular degeneration and Stargardt's macular dystrophy.10-14 Despite these advances, several practical contstraints impede translation of hES technology into clinical practice.Because of the plasticity and unrestricted growth potential of hES cells, one major concern of using transplanted hES cells in humans is the risk for teratoma formation and tumorigenesis, a challenge faced by all pluripotent stem cell platforms. [15][16][17] As recently reviewed, a number of animal studies have shown an increased risk for tumor formation arising from transplanted hES cells.18 Clearly, such an adverse eff ect must be addressed before hES technology can be off ered as an approved therapeutic product. Another risk inherent to allogeneic stem cell transplants is immune rejection. 19,20 Because hES cells are not "self " tissue, recipients would require immunosuppressive therapy similar to a patient receiving an organ transplant. For example, the Geron Phase I clinical trial stipulates that patients undergoing allogeneic hES cell transplants must receive immunosuppressive therapy for at least 4 months and be monitored for 15 years thereaft er to evaluate for rejection. 21Another concern for hES technology is resource availability. Given the absence of FDA-approved treatments based on hES cells, this topic has yet to be fully addressed. But as the research now shift s from the laboratory to the clinic in hopes of curing hundreds of diseases that aff ect millions of people, we will need to assess practically whether embryos derived from in vitro fertiliza...