After maternal predetermination gives way to zygotic regulation, a ground state is
established within the mammalian embryo. This tabula rasa for embryogenesis is present
only transiently in the preimplantation epiblast. Here, we consider how unrestricted
cells are first generated and then prepared for lineage commitment. We propose that
two phases of pluripotency can be defined: naive and primed. This distinction extends
to pluripotent stem cells derived from embryos or by molecular reprogramming ex vivo.