Stem cells are unique cell populations with the ability to choose between self-renewal and differentiation. Embryonic stem (ES) cells have the ability to form any fully differentiated cell of the body. To date, only three species of mammals have yielded long-term cultures of self-renewing ES cells- mice, monkeys, and humans. These cells have some special requirements to maintain their undifferentiated state in culture, e.g., presence of feeder cells, serum, or cytokines. Many scientific studies have tried to manipulate the growth and differentiation conditions with varied success. Studies of development in model systems, such as mice help our efforts to manipulate human stem cells in vitro. Data are now emerging that ES cells can be directed toward lineage-specific differentiation programs. On the basis of this property, it is likely that human ES cells will provide a useful differentiation culture system to study the mechanisms of human development. Recent advances in culturing ES cells and success in exploiting their pluripotency brings great hope for using human ES cell-based reparative therapy in future.