Primordial germ cells are the founder cells of the gametes. They arise at the earliest stages of development, and migrate to the genital ridges, where they join the somatic cells of the future gonad. The factors that determine their formation and migration are largely unknown. Primary culture of PGCs isolated at different times during their migration has allowed the analysis of some aspects of the control of their behaviour. The effects of culture medium conditioned by genital ridges suggested that both proliferation and migration of PGCs may be controlled in part by diffusible factors. Several purified growth factors are now known which affect PGC numbers in culture. These include positive and negative regulators of proliferation, survival factors, and chemotropic factors. During migration, PGCS must change their affinities for surrounding cells, and some aspects of this can be analysed in culture. For example, PGCs isolated at different stages in their migration show different capacities to adhere to fibronectin.