Stem cells are unspecialized cells capable of developing into a variety of specialized cells and tissues. Researchers are investigating how to culture them and control their differentiation, in the hope that they can be used to generate tissues or organs to treat a variety of diseases.

Stem cells can be derived from a variety of tissues. Adult, fetal, placental, umbilical, and embryonic stem cells all have different properties. Adult stem cells can be obtained from the blood, bone marrow, brain, pancreas, and fat of adult bodies. Embryonic stem cells are found in very early embryos, and can be obtained from "supernumerary" or "leftover" embryos donated by couples undergoing in vitro fertilization (IVF) treatment. Scientists have also derived embryonic stem cells from clonal human embryos.

Some researchers are particularly interested in using embryonic stem cells in research because they are totipotent, which means they are capable of developing into all tissue types. Other stem cells, such as adult stem cells, are generally thought to be multipotent, which means that particular types of adult stem cells are capable of developing into a limited number of tissue types. The exact potency of adult stem cells—both of individual types, and of adult stem cells taken as a group—is still an issue of debate among research scientists. Some recent studies suggest that adult stem cells may be as flexible as embryonic stem cells.

Medical researchers are also debating the relative safety of different kinds of stem cells. For instance, some researchers claim embryonic stem cells are difficult to control, because of their flexibility and tendency to proliferate rapidly. In fact, embryonic stem cells often behave similarly to cancer cells and have developed into tumors in some animal experiments. It has been argued that adult stem cells are much easier to control and less likely to behave in a tumor-like fashion and are thus more suitable for research.

Research using stem cells derived from adult tissues is almost universally supported. Research using stem cells derived from embryos created in the course of in vitro fertilization procedures is widely supported except by those who object to the destruction of human embryos.

The creation of human embryos by cloning is far more problematic. Many scientists consider it to be unacceptable, and a growing number of liberals and progressives are concerned that unregulated research cloning will open the door to reproductive cloning and inheritable genetic modification. Women's health advocates point out that the number of human eggs that would be needed for the medical use of research cloning would put at risk the health of the many women who would be needed as egg donors.