Hematopoiesis in adult animals is maintained by a small population of clonogenic, multipotent hematopoietic stem cells (HSC), which maintain throughout life the capacity to self-renew and to differentiate to give rise to progeny cells that ultimately generate all lineages of mature blood cells. In adult mice and humans, the majority of HSC are found in the bone marrow (BM); however, HSC are also constitutively present at low levels in the circulation. The frequency of HSC in the blood can be significantly increased through the use of “mobilizing” agents, including cytotoxic drugs and/or cytokines, which often act both to drive HSC proliferation and to induce HSC migration from the BM into the bloodstream. Yet despite the increasingly common clinical exploitation of HSC in bone marrow and mobilized peripheral blood progenitor cell transplantation, both the evolutionary rationale and the molecular mechanisms that underlie the remarkable migratory capacity of HSC remain largely unknown. Therefore, to begin to elucidate the mechanisms and regulators of these events, we have used parabiotic and transplantation models to characterize normal blood-borne HSC. Our data clearly demonstrate that HSC are constitutively present in the blood of untreated mice and maintain a cell surface phenotype in the blood highly similar to their BM counterparts. Blood-borne HSC in normal mice can engraft both irradiated and non-irradiated BM niches, and subsequently are phenotypically and functionally indistinguishable from endogenous, host-type cells. These data suggest that BM homing of transplanted HSC in irradiated recipients and HSC mobilization in cytokine-treated animals likely makes use of pre-existing pathways that support the constitutive recirculation of these cells in normal animals. Finally, to extend these data and begin to uncover factors likely to play a role in stimulating HSC migration in both normal and mobilized mice, we have employed cDNA microarray technology to compare global gene expression profiles of normal and pre-migratory BM HSC, and have thus identified multiple candidate genes, including cell cycle regulators, signaling molecules, and transcription factors, that may be involved in HSC expansion or in HSC retention in and/or egress from the BM. Taken together, these findings provide significant insight into the dynamic nature and function of HSC, and may ultimately suggest novel and improved strategies for clinical hematopoietic cell transplantation.