Human embryonic stem cells (HESC) are pluripotent stem cell lines derived from the inner cell mass (ICM) of human blastocyst-stage embryos. They are characterized by their unlimited capacity to self-renew in culture. In addition, they have a broad developmental potential, as demonstrated by their ability to form practically any cell type in vivo and in vitro. These two features have made HESC extremely important in basic and applied research. In addition, they may serve as a powerful tool for studying human development. HESC can recapitulate embryogenesis by expressing developmentally regulated genes and by activating molecular pathways as they occur in vivo. Moreover, they can be used to analyze the effect of specific mutations on particular developmental events and may enable us to identify critical factors that play a role in the processes of cell commitment, differentiation, and adult cell reprogramming. Thus, modeling human embryogenesis by the use of HESC may allow new insights into developmental processes, which would otherwise be inaccessible for research.