The thymic microenvironment provides critical signals for the proper development of T cells. The thymus is composed predominantly of epithelial cells of endodermal origin, but other cell types make up the thymus and facilitate thymic epithelial cell (TEC) and thymocyte development. This work focuses on molecular communication within the thymus. We have studied the zinc metalloprotease ADAM17, and its function in the thymus. ADAM17 is a member of the large ADAM (A Disintegrin And Metalloprotease) family, and it serves to cleave transmembrane proteins, shedding them from the cell surface. Through conventional knockout studies, it was discovered that ADAM17 regulates progression of thymocytes from the double negative (CD4-CD8-) stage to the double positive (CD4+CD8+) stage in adult animals, but through an unknown mechanism. The work in this dissertation focuses on three strategies to analyze the role of ADAM17 in thymocyte development. First, we have generated an in vitro model to analyze the effect of inhibiting ADAM mediated proteolytic cleavage of the Notch ligand Delta-like-1 (DLL1) and the subsequent effect on hematopoietic development. Second, we have utilized an ADAM17 conventional knockout model to assess its role in fetal T cell development. Third, we have generated a TEC specific ADAM17 knockout model by employing the Cre/Lox system to study the role of ADAM17 in the adult thymus. Our findings here demonstrate that ADAM mediated cleavage of the Notch ligand DLL1 affects hematopoietic lineage in vitro, that fetal and adult T cell development have differential requirements for ADAM17, and that ADAM17 may be acting on a non-TEC population in the thymus or extrathymically in mediating adult T cell development, while its presence on TECs regulates the expression of the key medullary TEC transcription factor Aire. These results provide new insights into thymic physiology and intrathymic molecular communication.