The promise and potential of human embryonic stem cell research evoke profound clinical enthusiasm1- 3; the embryonic human origins of such cells warrants an equally profound ethical concern. The ethical issues are not primarily matters of scientific fact nor of political belief. Consequently, these issues cannot adequately be addressed simply by reference to the biology of embryonic stem cells or the contemporary political context of stem cell research. To successfully make the case for developing the therapeutic potential of human embryonic stem cells, the biomedical community must engage these issues as genuine questions of morality and social policy. Just as an accurate understanding of stem cell biology is crucial to sound policy making, an accurate appraisal of the substantive and inseparable ethical issues is equally crucial. The biomedical community and society as a whole can answer these questions and justify the clinical promise of embryonic stem cell research but only by paying serious attention to the legitimate ethical issues.