Embryonic stem (ES) cell lines, which are derived from the inner cell mass of blastocysts, proliferate indefinitely in vitro, retaining their potency to differentiate into various cell types derived from all of the three embryonic germ layers: the ectoderm, mesoderm and endoderm. Establishment of human ES cell lines in 1998 has indicated the great potential of ES cells for applications in medical research and other purposes such as cell transplantation therapy. Careful assessment of safety and effectiveness using proper animal models is required before such therapies can be attempted on human patients. Monkey ES cell lines provide valuable models for such research.