Human embryonic stem cells (hESCs) are pluripotent, meaning that they have the ability to give rise to a wide range of cells belonging to all three germ layers (ectoderm, endoderm and mesoderm) as well as into germline lineage. The DNA code is more or less the same in all the cells of the human body. What differs is the way this code is utilized or, in other words, which combinations of genes that are active in a specific cell at a given time. The development of germ cells is a highly ordered process that begins during fetal growth and is completed in the adult. Due to their plasticity and potentially unlimited capacity for self-renewal, hESCs have been popular candidates for various cell therapies, including regenerative medicine and tissue replacement. The use of stem cells has provided a research tool to study development and disease in details never before possible.