This article is a tribute to the late Sir Peter Brian Medawar for his pivotal contribution in 1953 to the establishment of the field of Reproductive Immunology, with a brief and selective assessment of the progress made during the ensuing 50 years towards elucidation of the mechanisms responsible for the paradoxical survival of the conceptus as an intra-uterine allograft within the immunologically competent genetically alien female host. Medawar's succinct and stimulating theories have been central throughout the whole of this time and his basic conclusion, that the single most important factor ensuring the success of gestation is the anatomical separation of the fetus from its mother, remains substantially valid to this day. The extent to which other factors are of significance, particularly those relating to the relative roles of maternal adaptive and innate immune responses to the developing feto-placental unit, has yet to be fully defined.