Stem cells have two remarkable properties. They can either renew themselves or they can differentiate into one or more adult cell types. Stem cells derived from a human embryo appear to have an unlimited capacity to self-renew in cell culture, and they are also able to differentiate into hundreds of adult cell types. Human embryonic stem cell lines offer a platform technology that has the potential to elucidate the molecular mechanisms that determine adult cell fate, generate cellular models for discovery of new drugs, and create populations of differentiated cells for novel transplantation therapies. The National Institutes of Health (NIH) has identified some of the rate-limiting steps toward realizing this potential, and has forged funding initiatives to accelerate research progress. Given the remarkable potential, NIH support for research using stem cells is an important priority for the foreseeable future. Developmental Dynamics 236:3193–3198, 2007. Published 2007 Wiley-Liss, Inc.