The thymus is a complex cellular structure made up of several interdependent cell types and is the primary site for T cell development. A population of fetal thymic epithelial cells (TEC), marked by MTS20 and MTS24, when grafted in vivo can generate a functional thymus containing all thymic epithelial cells and is capab,e of supporting T cell differentiation. Further analysis using in vivo grafting experiments have determined the endoderm as the sole origin for all major thymic epithelial subsets. These findings suggest the possibility that a bipotent thmic epithelial progenitor cell (TEPC) gives rise to both cortical and medullary epithelial compartments. The first ai of this study was to address whether bipotent mouse TEPC give rise to both medullary and cortical epithelial cell populations and to begin to establish a model of TEC differentiation through ontogeny. Its second aim was to start to define condidtions for maintaining functionally undifferentiated RTEPC in vitro. Finally, as little is knowth about the genetic regulation of human thymic development and TEC differentiation, I hve used comparative analysis to investigate the similaarities in expression of key regulations of thymus development between human and mouse, which will aid in the translation of mouse thmic research to human. the main findings of this thesis are i) that a bipotent thymic epithelial progenitor cell population that contribute to both medullary and cortical epithelial cell compartments exists in vivo, but is at low frequency even by E12.5. ii) That a unipotent progenitor population committed to a cortical epithelial cell fate is also present as early as E11.5. iii) That E11.5 TEC can be propagated in vitro in semi defined conditions, but appear to revert ro an early endodermal phenotype on prolonged culture and iv) that the genetic program regulating thymus organogenesis appears to be conserved between mouse and human. In addition I have defined the exact time of haematopoietic cell entry into the human thymus, and the time of entry of mesenchymal cells and the mesenchymal distribution pattern throughout human thymic ontogeny. I also establish the time of onset of differentiation and maturation of hTEC, and using known mouse TEPC markets, show the existence of a population of Claudin4+Ueal+ hTEC at week 8, presumed equivalent to the precursor cells for the AIRE1+ subpopulation of medullary TEC in the mouse.