The groundbreaking development of embryonic stem (ES) cells from the mouse in the early 1980s generated a sustained expansion of research worldwide into their exploitation, leading to the genetic engineering of the mouse to an unprecedented level. This ES cell-based methodology has contributed normously to the understanding of biological processes in the context of the whole animal. With the advent of ES cells from other species including the human, ES cell biology will remain for the foreseeable future a crucial and growing area of research, with far-reaching implications for developmental and comparative biology, and for uman health