During embryogenesis there is a sequential, temporal appearance of increasingly more-complex hematopoietic cells beginning with unipotential progenitors, proceeding to multipotential (myeloid, erythroid and lymphoid) progenitors and culminating with adult-repopulating hematopoietic stem cells. Current research has established an important role for the aorta–gonads–mesonephros region of the mouse embryo in the generation of multipotential progenitors and hematopoietic stem cells. Comparisons of normal and hematopoietic-cell-mutant mouse embryos have revealed several genes pivotal in hematopoietic stem cell generation/function. Other genes have been implicated in the critical generation of lymphoid lineage potential. Thus, an understanding of the cellular and molecular interactions within the midgestation aorta–gonads–mesonephros region offers insight into the mechanisms of hematopoietic lineage specification during ontogeny and perhaps will lead to a more complete knowledge of the adult hematopoietic system.