The thymus is an organ that is located above the heart and plays a vital role in the establishment of the immune system. Particularly before and during the first years after birth, the thymus is needed to support the development of a critical type of white blood cells that plays a central role in the immune system, called T lymphocytes. These immune cells are generated from blood forming stem cells that undergo a series of discrete developmental stages within the specific microenvironment of the thymus. Within this organ, also other immune cells are present, such as dendritic cells, natural killer cells, B cells and innate lymphocytes, and some of these have important roles in fine-tuning the proper development of T lymphocytes. These immune cells may also develop in thymus from the same precursors cells as the T cells, but could also colonize the thymus as mature cells after being generated in the bone marrow. This, however, is still unclear and we wish to unravel this in this proposal using state-of-the-art technologies. As such, we will obtain new insights into the establishment of the immune system across development in human. These results will therefore have a strong translational impact by paving the way for improving immune reconstitution and generating cell therapies.