Mouse embryonic stem (ES) cells are pluripotent cells derived from the early embryo and can be propagated stably in undifferentiated statein vitro. They retain the ability to differentiate into all cell types found in the embryonic and adult bodyin vivo, and can be induced to differentiate into many cell types under appropriate culture conditionsin vitro. Using these properties, people have set up various differentiated systems of many cell types and tissuesin vitro. Through analysis of these systems, one can identify novel bioactive factors and reveal mechanisms of cell differentiation and organogenesis. ES cell-derived differentiated cells can also be applied to cell transplantation therapy. In addition, we summarized the features and potential applications of human ES cells.