Embryonic stem (ES) cells derived from preimplantation embryos at the blastocyst stage are unique in their unlimited self-renewal ability and pluripotency allowing their differentiation into any cell type. For these reasons ES cells are considered as a perfect material for basic research on their differentiation capacities, and also for the studies devoted to the development of novel therapies. This article focuses at the mechanisms regulating ES cells pluripotency, self-renewal, and also influencing their differentiation. It also describes alternative methods of the derivation of pluripotent stem cell lines, which do not require embryo destruction and thus do not raise ethical issues. A special attention is paid to the development of ES cell culture.