Pluripotency, the capacity to generate all cell types of the body, lies at the foundation of development in mammals. In 1981 scientists discovered that pluripotency could be maintained in the laboratory in cells called embryonic stem cells. Study of these unique cells over the past 30 years has uncovered the molecular machinery that governs pluripotency. These studies have also highlighted significant differences between ES cells from rodents and from humans. So what makes an authentic embryonic stem cell?