The T cell compartment is continuously replenished by a renewable source of stem cells. In the adult, bone marrow-derived stem cells seed the thymus and initiate a developmental program that requires a series of incompletely understood signals that are normally provided by the thymus. Failure to recapitulate this process in simple in vitro cultures has hampered efforts to fully characterize these unique signals. In this issue of Immunity, Schmitt and Zúñiga-Pflücker describe a simple in vitro culture system that is able to generate mature T cells from fetal liver stem cells by expressing the Notch ligand Delta-1 on the OP9 stromal cell line. This finding should greatly enhance efforts to study T cell development and may provide a tool for generating defined T cell populations in vitro.