Embryonic stem (ES) cells are a primitive cell type derived from the inner cell mass (ICM) of the developing embryo. When cultured for extended periods, ES cells maintain a high telomerase activity, normal karyotype and the pluripotential developmental capacity of their ICM derivatives. Such capacity is best demonstrated by mouse ES cells which can contribute to all tissues of the developing embryo following either their injection into host blastocysts or tetraploid embryo complimentation (for a review see Robertson). For both practical and ethical reasons it is not possible to inject human ES cells into blastocysts for the development of a term fetus. However, when injected beneath the testicular capsule of severe combined immunodeficient (SCID) mice, human ES cells form teratomas comprising tissue representatives of all three embryonic germ layers (ectoderm, mesoderm and endoderm) thus attesting to their pluripotency. Based upon morphological criteria, neuronal, cardiac, bone, squamous epithelium, skeletal muscle, gut and respiratory epithelia are readily identifiable within the human ES-cell-derived teratomas. With the demonstrated capability to isolate and maintain pluripotent human ES cells in vitro, their ability to give rise to tissue representatives of all three embryonic germ layers and the technical advances made possible by research on mouse ES cells, a rapid increase in human ES cell research aimed at drug discovery and human cell and gene therapies has occurred. Indeed in the mouse, dissociated embryoid bodies (EBs) have already been demonstrated capable of repopulating the haematopoietic system of recipient animals (for a review see Keller) and mouse ES cells are currently being used in attempts to repair mouse neural degenerative lesions.