Cellular and gene therapy are obvious approaches to correct rare genetic disorders and, in the future, degenerative conditions. Both methods rely on the ability to identify and genetically modify stem cells in vitro, and on the reproducibility of in vivo colonization by the manipulated cells. The basic processes that are involved in the generation of haematopoietic cells in the embryo have been described in the past ten years, as a result of interdisciplinary approaches. These efforts have led to the identification of two independently generated types of haematopoietic progenitor cell, which differ in their potential for self-renewal and differentiation. These two populations might become key tools to understand the properties of stem cells.