Pluripotent stem cells (PSCs) have been classified into two distinct states: a primitive, naive LIF-dependent state represented by murine ESCs, and a primed bFGF-dependent state observed in murine and rat epiblast stem cells (EpiSCs). The vast similarities between EpiSCs and human ESCs suggest that, despite their blastocyst origin, human ESCs exist in a primed pluripotent state. Recent findings demonstrate that the naive and primed pluripotent states are interconvertible, even in human cells, and hint that growth factor-mediated Nanog expression may be an important factor regulating the balance between them.