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Therapeutic cloning has become possible since the discovery that nuclei from somatic cells of adult animal tissue can successfully be used for cloning and the fact that human embryonic stem cell lines have been established from preimplantation embryos. When nuclei from healthy tissue of a patient are transplanted into enucleated oocytes, these oocytes can be artificially activated so that embryos develop from which embryonic stem cells of the donor can be derived. These embryonic stem cells can be cultured as permanent lines in unlimited numbers and remain pluripotent, i.e. they can be induced to differentiate into the required cell type by adding one or more specific factors. These cells can then be transplanted back into the patient suffering from either a lack or dysfunction of these cells. This approach prevents the rejection of the transplanted cells by the patient's immunological system. As this type of cloning has a very low efficiency, a large number of unfertilized donor oocytes is required. It is questionable whether enough donors are or will be available for this purpose. The cultured cells must satisfy certain conditions before they can be used for transplantation. They must be checked for chromosomal abnormalities, and a complete differentiation of the embryonic stem cells into the cells types needed by the patient is necessary as after the transplantation, undifferentiated stem cells will form teratomas. Furthermore, it is difficult to ensure that the cells end up in the right place and to ensure that they fully integrate into the existing tissue to form functional connections. Due to this array of technical problems the question remains as to whether therapeutic cloning will become feasible in the near future.