T lymphocyte development in the thymus is a tightly regulated stepwise process. The identification and characterization of the earliest T lineage progenitors and their downstream progeny now enables the study of important cellular and molecular mechanisms that control and regulate T lineage commitment and differentiation. Significant progress has been made recently on the developmental relationships of the various cells with T cell progenitor activity identified in mouse bone marrow, blood and thymus, and on the molecular regulation of progenitor homing to the thymus. The essential role of Notch-1 signalling in intrathymic T lineage commitment and subsequent T cell development has been clearly documented.