The nature of the molecular interactions provided by the thymus that predicate T cell development remains obscure. In this thesis, I demonstrate that the bone marrow (BM) stromal cell line OP9, when made to express the Notch ligand Delta-like-1 (Dll1), loses its ability to support B cell lymphopoiesis, and acquires the capacity to induce the development of CD4 CD8 double- and single-positive T cells from various hematopoietic progenitor cells. Both gammadelta-TCR + and alphabeta-TCR+ T cells are generated, and CD4- CD8+ TCRhi cells produce gamma-interferon following CD3/TCR stimulation. Dll1 expressed on OP9 cells provides the necessary signals to induce T cell commitment, stage-specific progenitor expansion, TCR gene rearrangement, and T cell differentiation in-vitro. A normal program of T cell differentiation was also observed from embryonic stem cells (ESCs) cultured on these OP9 cells, which expressed multiple T lineage-associated genes in response to Notch receptor-Dll1 interactions. Furthermore, ESC-derived T cell progenitors effectively reconstituted the T cell compartment of immunodeficient mice, and were capable of generating an antigen specific response to a viral challenge.Using this culture system, I demonstrate that a substantial proportion of early thymocytes retain NK cell lineage potential, and that Notch signals act prior to T cell lineage commitment to maintain T cell lineage specification in early thymocytes. Furthermore, Notch receptor-ligand interactions are shown to be critical throughout T cell development. Thus, it is likely that the expression of Delta-like ligands in the thymus underpins its unique ability to promote T cell lineage commitment and differentiation.