Pluripotency is defined as the potential of a cell to differentiate into cells of the three germ layers: endoderm, mesoderm and ectoderm. In vivo, the presence of pluripotent stem cells is transient during the very early embryo. However, immortal cell lines with the same properties can be obtained in vitro and grown indefinitely in laboratories under specific conditions. These cells can be induced to differentiate into all the cell types of the organism through different assays, thereby proving their functional pluripotency. This review focuses on the pluripotent stem cells of mammals, giving special attention to the comparison between mouse and human. In particular, embryonic stem cells, epiblast-derived stem cells, primordial germ cells, embryonic germ cells, very small embryonic-like cells and induced pluripotent stem cells will be compared in terms of the following: in vivo specification and location; surface and intracellular markers; in vitro dependence on growth factors; signal transduction pathways; epigenetic characteristics; and pluripotency genes and functional assays.