When male germ line stem cells are transplanted from the testis of a fertile donor animal to the testis of an infertile recipient they can establish donor-derived spermatogenesis in the recipient testis, and the resulting sperm can transmit the genotype of the donor to the offspring of the recipient. Germ cell transplantation provides a bioassay to study the biology of these stem cells, to develop systems for spermatogonial stem cell isolation and culture, to examine defects in spermatogenesis and to correct male infertility. Although most widely studied in rodents, germ cell transplantation has been applied to larger mammals, including primates. A potential clinical application is restoration of fertility in patients that underwent cytotoxic treatments for cancer. As an alternative to transplantation of isolated germ cells to a recipient testis, ectopic grafting of testis tissue from diverse mammalian donor species, including primates, into a mouse host represents a novel possibility to study spermatogenesis, to investigate the effects of toxins or drugs with the potential to enhance or suppress male fertility, and to produce fertile sperm from immature donors. Therefore, transplantation of germ cells or xenografting of testis tissue are uniquely valuable approaches for the study, preservation and manipulation of male fertility in mammalian species.