This special issue adopts a comparative approach to the politics of reproduction in twentieth-century France and Britain. The articles investigate the flow of information, practices and tools across national boundaries and between groups of experts, activists and laypeople. Empirically grounded in medical, news media and feminist sources, as well as ethnographic fieldwork, they reveal the practical similarities that existed between countries with officially different political regimes as well as local differences within the two countries. Taken as a whole, the special issue shows that the border between France and Britain was more porous than is typically apparent from nationally-focused studies: ideas, people and devices travelled in both directions; communication strategies were always able to evade the rule of law; contraceptive practices were surprisingly similar in both countries; and religion loomed large in debates on both sides of the channel.