T cells typically differentiate via a series of coordinated steps within the highly specialized microenvironment of the thymus. Traditionally, human T-lymphopoiesis in vitro has been studied using the hybrid human/mouse fetal thymic organ culture system. Pioneering work by McCune et al. devised a method to examine human T cell development in vivo in relation to HIV-1 using the SCID/hu (thy/liv) model. This was followed by models that better reflected the ability of human hematopoietic cells to home and differentiate within the mouse host without human fetal tissues; however, human T cell development in these animals was poor. In this chapter, we outline a procedure to generate human progenitor T (proT) cells in vitro from umbilical cord blood-derived hematopoietic stem cells using the OP9-DL1 cell system; in addition, we describe the method used to examine the engraftment of in vitro-derived proT cells into immunodeficient mouse strains.