T-cell development is initiated when CD34+ pluripotent stem cells or their immediate progeny leave the bone marrow to migrate to the thymus. Upon arrival in the thymus the stem cell progeny is not yet committed to the T-cell lineage as it has the capability to develop into T, natural killer (NK) and dendritic cells (DC). Primitive hematopoietic progenitor cells in the human thymus express CD34 and lack CD1a. When these progenitor cells develop into T cells they traverse a number of checkpoints. One early checkpoint is the induction of T-cell commitment, which correlates with appearance of CD1a and involves the loss of capacity to develop into NK cells and DC and the initiation of T-cell receptor (TCR) gene rearrangements. Basic helix-loop-helix transcription factors play a role in induction of T-cell commitment. CD1a+CD34+ cells develop into CD4+CD8α+β+ cells by upregulating first CD4, followed by CD8α and then CD8β. Selection for productive TCRβ gene rearrangements (β selection) likely occurs in the CD4+CD8α+β− and CD4+CD8α+β+ populations. Although the T and NK-cell lineages are closely related to each other, NK cells can develop independently of the thymus. The fetal thymus is most likely one site of NK-cell development.