Abstract: The present invention relates generally to novel thymic cellular populations and, more particularly, to novel thymic epithelial cellular populations. Most particular, the present invention is directed to novel thymic epithelial progenitor cell populations. The cellular populations of the present invention are useful in a wide range of clinical and research settings including, inter alia, the in vitro or in vivo generation of thymic epithelial cell populations and the therapeutic or prophylactic treatment of a range of conditions via the administration of these cells. Also facilitated is the design of in vitro based screening systems for testing the therapeutic impact and/or toxicity of potential treatment or culture regimes to which thymic epithelial cells may be exposed.