Haematopoietic stem cells (HSCs) emerge during embryogenesis and maintain hematopoiesis in the adult organism. Qualitative and quantitative assessment of HSCs can only be performed functionally using the in vivo long-term repopulation assay. Due to the lack of such data, little is known about the development of HSCs in the human embryo, which is a prerequisite for the development of new therapeutic strategies. Employing the xenotransplantation assay, I have performed here the spatio-temporal mapping of HSC activity within the human embryo and have shown that human HSCs emerge first in the aorta-gonad-mesonephros (AGM) region, specifically in the ventral wall of the dorsal aorta, and only later appear in the yolk sac, liver and placenta. Human AGM region HSCs transplanted into immunodeficient mice provide long-term high-level multilineage haematopoietic repopulation. These cells, although present in the AGM region in low numbers, exhibit a very high self-renewal potential. A single HSC derived from the AGM region generates around 600 daughter HSCs in primary recipient mice, which disseminate throughout the entire recipient bone marrow and are retransplantable. These findings highlight the vast regenerative potential of the earliest human HSCs and set a new standard for in vitro generation of HSCs from pluripotent stem cells for the purpose of regenerative medicine. I have also established a preliminary immunophenotype of the earliest human HSC. These data will be useful for my future studies on the mechanisms underlying the high potency of human embryonic HSCs and on the characterisation of embryonic HSC niche.