The derivation of murine embryonic stem (mES) cell lines was reported for the first time in 1981 (Nature, 1981; 292:154-156; Proc Natl Acad Sci U S A, 1981; 78:7634-7638), and they have since proved to be a very useful tool with which to study mammalian development, which is characterized by pluripotency and differentiation. About 20 years later, the successful generation of human embryonic stem (hES) cell lines was described (Science, 1998; 282:1145-1147). Although mES and hES are derived from mammals, they cannot be looked at as being one and the same. While basic information for hES can be derived from mES, such information does not correspond on a one-to-one basis. This review gives an overview of the characteristics of embryonic stem cells with the main focus on the similarities and differences between human and mES cells.