Stem cells could help medicine in three general ways: cell-based therapies, drug discovery and basic knowledge. Cell therapies would use stem cells, or cells grown from stem cells, to replace or rejuvenate damaged tissue. Scientists also want to use stem cells to understand disease and find drugs that might treat it.

Embryonic stem cells could be used to make more specialized tissues that have been lost to disease and injury. For tissues that are constantly replaced, like blood and skin, stem cells would probably be replaced directly. Researchers are also exploring ways to use stem cells to treat diabetes, Parkinson's disease, spinal cord injury, heart disease and vision and hearing loss, among others.

As of April 2007, however, no therapies using cells derived from embryonic stem cells have been tested in humans. The efficacy of stem cell therapies depends on the introduced cells arriving where they are needed and either replacing or rejuvenating damaged cells. They should not contain undifferentiated embryonic stem cells, and either the cells, the patient or both should be treated so that the patient's immune system will not attack the transplant