This chapter reviews molecular mechanisms that control germline formation through a complex cascade of gene activation. In mammals, primordial germ cells (PGCs) are derived from the proximal epiblast during early embryogenesis. Interestingly, although both FRAGILIS and STELLA are differentially expressed in PGCs, neither appears to be essential for PGC specification. In general, migration of PGCs from primitive streak to genital ridges is believed to be governed by chemotactic cytokines, cell surface receptors, and cell adhesion factors. Until the colonization of the genital ridges, XX and XY PGCs are indistinguishable in terms of morphology and behavior. Mammalian male sex determination is initiated by sex-determining region Y (SRY) expression in XY genital ridges, which triggers Sertoli cell differentiation in supporting cell precursors. Germ-cell colonization of the gonads is followed by sex determination. Expression of sex-specific genes in somatic tissues initiates molecular events that lead to testis or ovary development.