Since human embryonic stem cells (hESCs) were first isolated and successfully cultured in vitro, the pluripotent potential of hESCs has been underestimated. The pluripotency of mouse embryonic stem cells (mESCs) can be categorized as naïve and primed, depending on their corresponding in vivo developing phases. mESC morphology differs at distinct pluripotent states, which differ in signaling dependence, gene expression, epigenetic features, and developmental potential. hESCs resemble mouse stem cells at primed pluripotency, and consequently are believed to correspond to a later developmental stage in vivo than mESCs. Nevertheless, recent studies indicate that a naïve state of pluripotency may exist in hESCs, and the pluripotency of hESCs also can be enhanced by genetic modification or optimized culture systems. These findings provide novel insight into the properties and differentiation potential of hESCs. Here, we review the recent advances in characterization of ESC states and investigate the mechanisms regulating hESC pluripotency.