Primordial Germ Cells (PGCs) arise in the mouse embryo as a small population of cells some way from the gonad anlagen. In order for the embryo to develop into a fully fertile adult animal the PGCs must increase in number and reach the gonad. Mutations causing sterility in the mouse have identified some of the genes involved in regulating PGC development and some of these genes have been molecularly cloned. Similarly, mutations affecting the development and differentiation of PGC-derived tumors (teratomas and teratocarcinomas) have been identified in certain strains of mice and these identify genes involved in the normal growth and differentiation of PGCs. These studies should help to define the role of growth factors in PGC development and in the development of germ-cell-derived tumors.