Stem cells represent natural units of embryonic development andtissue regeneration. Embryonic stem (ES) cells, in particular, possess anearly unlimited self-renewal capacity and developmental potential todifferentiate into virtually any cell type of an organism. Mouse ES cells,which are established as permanent cell lines from early embryos, can beregarded as a versatile biological system that has led to major advances incell and developmental biology. Human ES cell lines, which haverecently been derived, may additionally serve as an unlimited source ofcells for regenerative medicine. Before therapeutic applications can berealized, important problems must be resolved, such as, ethical issuessurrounding the derivation of human ES cells from in vitro fertilizedblastocysts, immunological rejection of transplanted ES cells. Oneapproach to overcome transplant rejection of human embryonic stem (ES)cells is to derive ES cells from nuclear transfer of the patient's own cells.Somatic cell nuclear transfer (SCNT) and lastly the number of human EScell lines available for research may also be insufficient to adequatelydetermine their therapeutic potential. Recent molecular and cellularadvances with mouse ES cells, however, portend the successful use ofthese cells in therapeutics. This review therefore focuses both on mouseand human ES cells with respect to in vitro propagation, differentiationand cloning as well as their use in basic cell and developmental biologyand toxicology and presents prospects for human ES cells in tissueregeneration and transplantation.