Primordial germ cells (PGCs), the precursors of the germ cells, arise from pluripotent epiblast cells during gastrulation in the mouse embryo. During early PGC development, there are a series of cellular events, each of which is important in the acquisition of totipotency. Pluripotent stem cells, such as embryonic stem (ES) cells, are a potential source of PGCs in vitro. Accumulating evidence suggests that ES cells can differentiate into PGCs via either embryoid body formation or monolayer cell culture. However, it remains unclear whether these in vitro-derived PGCs properly accomplish all developmental processes during PGC specification in vivo. This article reviews current studies of PGC specification both in vivo and in vitro, shelding light on the significance of PGC specification, and provides a guideline for verification in the process of PGC production in vitro.