Human embryonal hematopoiesis is characterized by a primitive program capable of generating transient precursors after which true HSCs are generated in a definitive wave. These HSCs further expand in the FL before migrating to the fetal BM which supports hematopoiesis throughout postnatal life. HSCs then egress as TSPs into the bloodstream in order to colonize the thymus where the NOTCH1/DLL4 axis induces the T cell program. T cell development occurs through sequential progression through distinct intermediate stages in which germline rearrangements of TCR loci primarily instructs the bifurcation of the αβ and γδ T cell lineages. HSC: hematopoietic stem cell; AGM: aorta-gonad-mesonephros; YS: yolk sac; FL: fetal liver; BM: bone marrow; CMJ: cortico-medullary junction; TSP: thymus seeding precursor; TEC: thymic epithelial cell; ETP: early T cell progenitor; pro-T: progenitor T cell; CTP: committed T cell precursor; ISP: immature single positive; ISP28⁺: β-selected ISPs; DP: double positive; SP4/8: CD4/CD8 single positive; γδ: γδ T cell. (Created with BioRender.com).