Stem cells are undifferentiated cells that can develop into specialized tissue types. Researchers are investigating how to isolate and culture them, and control their differentiation, in the hope that they can be used to treat and understand a variety of diseases.

Stem cells can be derived from a number of cellular sources: adult, fetal, and placental tissues; umbilical cord blood; and embryos. Stem cells from these different sources have different properties.

Adult stem cells can be obtained from the bodies of adults and children, and until recently considered multipotent, which means that particular adult stem cells can develop into specific tissue types. Adult stem cells have been used in therapies such as bone marrow transplants for years.

Embryonic stem cells are found in early embryos. They are pluripotent, which means they can develop into all tissue types and be cultured as stem cell "lines." No therapies have been developed from human embryonic stem cells, which were first isolated in 1998.

In recent years, new methods of cellular reprogramming have enabled the derivation of so-called induced pluripitent stem (iPS) cells, which seem to have the full powers of embryonic stem cells but are from adult body cells.

Human embryonic stem cell research is controversial because it destroys embryos. Most investigations use embryos created but not used for in vitro fertilization (IVF) treatment. Some scientists have worked to derive human embryonic stem cells using a cloning technique called research cloning, which raises a separate set of troubling questions.