Thymus, the main organ for T lymphopoiesis, requires a permanent influx of progenitors from bone marrow (BM) or fetal liver. An essential question relating to early T-cell development is the identification of the progenitor population which actually homes to the thymus. Recent findings have shown that human multipotent progenitor/stem cells expressing CD34 have the capacity to differentiate into T cells when introduced into a thymic environment. More mature CD34+ bone marrow cells coexpressing CD7 and having a poor myeloid differentiation capacity can also efficiently differentiate into T cells in vitro. These lymphoid committed precursors might be the true thymic repopulating cells. In the thymus, cells with a similar CD34+7+ phenotype include the most primitive thymocyte precursors. CD34+ thymocytes have no myeloid differentiation potential, but may include precursors for natural killer (NK) cells. Interleukin-7 (IL7) is a potent in vitro growth factor for CD34+ thymocytes. Whereas current data do not support a crucial role for IL2, patients with IL2 receptor gamma chain (IL2R gamma) deficiency lack T- and NK cells. The recent demonstration that IL2R gamma is part of the receptor for IL7 strongly suggests that this cytokine plays an essential role in in vivo T lymphocyte and NK development.