Embryonic stem (ES) cells are cells derived from the early embryo that can be propagated indefinitely in the primitive undifferentiated state while remaining pluripotent; they share these properties with embryonic germ (EG) cells. Candidate ES and EG cell lines from the human blastocyst and embryonic gonad can differentiate into multiple types of somatic cell. The phenotype of the blastocyst-derived cell lines is very similar to that of monkey ES cells and pluripotent human embryonal carcinoma cells, but differs from that of mouse ES cells or the human germ-cell-derived stem cells. Although our understanding of the control of growth and differentiation of human ES cells is quite limited, it is clear that the development of these cell lines will have a widespread impact on biomedical research.