A considerable amount of data has been accumulated on the developmental origins of the mammalian haematopoietic system, particularly within the intraembryonic aorta-gonad-mesonephros (AGM) region. Here, Elaine Dzierzak, Alexander Medvinsky and Marella de Bruijn review cellular and molecular data on mouse developmental haematopoiesis and discuss the implications for the long-held notion of the yolk sac origin of the adult haematopoietic system.