The recent development of embryonic stem (ES) cells from human blastocysts has the potential to revolutionize many of our approaches to human biology and medicine. Continued objection to the use of human ES cells on ethical grounds may inhibit progress or defer this opportunity indefinitely. It is essential that the ethical discussion proceed on a sound scientific basis. The ethical controversy surrounding human ES cells concerns their origin from human blastocysts and the perception of their developmental potential. It is likely that the worldwide requirement for human ES cells will be met by the development of a small number of cell lines, as has been the case in the mouse; current rates of success for human ES cell establishment suggest that only a modest number of embryos will be required to achieve this goal. It is in the public interest that human ES cell lines be derived under circumstances that will enable their widespread distribution with minimum encumbrances to academic researchers throughout the world. In considering the developmental potential of ES cells, an important distinction exists between pluripotentiality, or the ability to develop into a wide range of somatic and extraembryonic tissues, and totipotentiality, the ability of a cell or collection of cells to give rise to a new individual given adequate maternal support. There is no evidence that ES cells from any species can give rise to a new individual except when combined with cells which are the immediate progeny of a zygote. These developmental limitations of ES cells appear to relate to their inability to undergo axis formation and to generate the body plan. Alternatives to blastocyst-derived ES cells include embryonic germ cells, adult tissue stem cells, transdetermination of committed somatic cells, and therapeutic cloning. These research areas are complimentary and synergistic to ES cell research and it is premature and counterproductive to suggest that one avenue should be pursued in preference to another. The combination of cloning and ES cell technology has the potential to address many important issues in transplantation medicine and research, but a better understanding of the reprogramming of somatic cells is required before we can regard ES cells derived from normal and nuclear transfer blastocysts as equivalent.