T-cell development in the thymus is a complex and highly regulated process. During the process of differentiation from multipotent progenitor cells to mature T cells, proliferation, restriction of lineage potential, TCR gene rearrangements and selection events occur, all accompanied by changes in gene expression. A comprehensive understanding of thymocyte differentiation remains to be established. Two related, key issues have received much attention recently: the nature of the thymus seeding cell and the regulation of T-cell lineage commitment. Here we review the perspectives of different researchers working both on murine and human T-cell development and argue that a true T-cell commitment factor might not be required because of the unique properties of the thymus.