Discussion of the bioethics of human stem cell research has transitioned from controversies over the source of human em-bryonic stem cells to concerns about the ethical use of stem cells in basic and clinical research. Key areas in this evolving ethical discourse include the derivation and use of another human embryonic stem cell-like stem cells that have the capacity to differentiate into all types of human tissue and the use of all types of stem cells in clinical research. Each of these issues is discussed as I summarize the past, present, and future bioethical issues in stem cell research. The main bioethical issues associated with human stem cells involve their derivation and use for research. Although there are interesting ethical issues surrounding the collection and use of somatic (adult) stem cells from aborted fetuses and umbilical cord blood, the most intense controversy to date has focused on the source of human embryonic stem (hES) cells. At present, new ethical issues are beginning to emerge around the derivation and use of other hES cell-like stem cells that have the capacity to differentiate into all types of human tissue. In the near future, as the stem cell field progresses closer to the clinic, additional ethical issues are likely to arise concerning the clinical translation of basic stem cell knowledge into reasonably safe, effective, and accessible patient therapies. This Review summarizes these and other bioethical issues of the past, present, and future of stem cell research.