The thymus provides a unique environment for the development of T lymphocytes from bone marrow‐derived progenitor cells. Several environmental factors have been identified that influence the development of T cells in the thymus. In particular, the Notch pathway has emerged as critical for the induction of T‐lineage commitment and differentiation. Until recently, however, the precise nature of the thymus‐derived signals that drive T‐cell development were unclear, and the only reliable in vitro culture system that supported T‐cell differentiation required the use of thymus organ cultures. Here, we discuss recent advances in the identification of critical Notch receptor ligands that have facilitated the development of a simple in vitro model for the differentiation of T cells ‘in a dish’, providing an alternate approach for studying T lymphopoiesis.