Embryonic stem cells (ESCs) are the pluripotent cell population derived from the inner cell mass of pre-implantation embryos and are characterised by prolonged self-renewal and the potential to differentiate into cells representing all three germ layers both in vitro and in vivo. Preservation of the undifferentiated status of the ESC population requires the maintenance of self-renewal whilst inhibiting differentiation and regulating senescence and apoptosis. In this review, we discuss the intrinsic and extrinsic factors associated with self-renewal process, together with possible signalling pathway interactions and mechanisms of regulation.