Human embryonic stem cells derived from the pluripotent inner cell mass of the pre-implantation embryos, Human embryonic stem cells express typical pluripotent stem cell markers such as octamer-binding transcription factor 3/4 (OCT3/4), stage specific embryonic antigens 3 and 4 (SSEA-3 and SSEA-4), TRA-1-60, TRA-1-81 and alkaline phosphatase, process high levels of telomerase activity and show normal karyotype. Human embryonic stem cells have capacity to differentiate into cell types of all three germ layers (endoderm, mesoderm, and ectoderm) under in vitro and in vivo conditions. Consequently, human embryonic stem cells hold great promise in understanding of early human embryology and for developing the cell replacement strategies for the treatment of human diseases.