While in vitro models exist to study human T cell development, they still lack the precise environmental stimuli, such as the exact combination and levels of cytokines and chemokines, that are present in vivo. Moreover, studying the homing of hematopoietic stem (HSC) and progenitor (HPC) cells to the thymus can only be done using in vivo models. Although species-specific differences exist, "humanized" models are generated to circumvent these issues. In this chapter, we focus on the humanized mouse models that can be used to study early T cell development. Models that study solely mature T cells, such as the SCID-PBL (Tary-Lehmann et al., Immunol Today 16:529-533) are therefore not discussed here, but have recently been reviewed (Shultz et al., Nat Rev Immunol 12:786-798).