The most influential understanding of human generation to emerge from classical antiquity belongs to the great physician of the Roman Empire, Galen of Pergamum. This chapter outlines and analyses the generative parts of Galen’s medical system in dialogue with pre-existing medical views and philosophical debates on the subject, on the one hand, and with the broader shape and substance of his overall project and Roman imperial society, on the other. It argues that the modern focus on Galen’s decisive embrace of what has become known as the ‘two-seed theory’ of generation, in which both women and men, rather than just the latter, produce seed, is misleading. The approaches current in second-century AD Rome had more in common than divided them, including the crucially shared assumption that females both contribute to their offspring and are inferior. The understandings of conception, fetal formation, growth and nourishment, of the mechanics of birth, and lactation, were also generally agreed on, though Galen provided a particular version of this story. It was this version that went on to dominate the Greek medical encyclopaedias and schools of the later Roman Empire, and would be translated, most extensively, into Syriac and Arabic.