Abstract The thymus is the primary lymphoid organ responsible for the generation of functional T cells, and is therefore critical for generation and maintenance of adaptive immunity. Thymic degeneration during aging, termed involution, results in a dramatic drop in the production of naive T cells, and is a major contributing factor to immune senescence. Despite the importance of this subject for human health, the molecular and cellular mechanisms operating in the postnatal thymus that mediate thymic homeostasis and involution are largely unknown. As a result, many aspects of thymic involution are poorly understood and/or controversial. For example, the relative contributions of changes in thymic stromal cells, hematopoietic stem cells, and physiology to thymic involution are topics of current debate. In the current project, we provide evidence that the down regulation of a single gene expressed only in thymic epithelial cells, the Foxn1 transcription factor, is sufficient to induce rapid and premature thymic involution. The transcription factor Foxn1 is necessary and sufficient for t fetal thymic epithelial cell (TEC) differentiation, and is widely (although not ubiquitously) expressed in postnatal TECs; however, a function in the postnatal thymus has not been previously idenified. Using a novel allele of Foxn1, Foxn1lacZ, we show that reduction of Foxn1 expression in the postnatal thymus causes a postnatal thymic degeneration phenotype that recapitulates most or all stromal and thymocyte-specific defects characteristic of aging-associated involution. This phenotype is associated with decreased Foxn1 gene expression, and provides functional evidence that Foxn1 is required in postnatal TECs to maintain the postnatal steady-state thymus. The requirement for Foxn1 is extremely dosage-sensitive, with small changes in Foxn1 levels having large effects on thymus phenotypes, and TEC subsets that express higher Foxn1 levels being most sensitive to its down-regulation. Based on these and other data, we propose that different TEC subsets require specific levels of Foxn1 for their differentiation, proliferation, and maintenance, and that decreasing Foxn1 levels with age directly contribute to thymic involution both by reducing TEC proliferation and by reducing the capacity to generate specific TEC subsets. These TEC-specific defects are then causative for most involution effects. Thus, our data indicate that we have identified Foxn1 and its function within TECs as a critical regulatory node in the complex network of the postnatal thymus. We propose three specific aims to test the hypothesis that down regulation of Foxn1 gene expression is both necessary and sufficient to induce thymic involution. Aim 1 will define the expression of Foxn1 in specific TEC populations in the postnatal thymus, and test whether its down regulation is both necessary and sufficient to trigger thymic involution. Aim 2 will test whether Foxn1-mediated regulation of proliferation in TECs is a critical component of involution. Aim 3 will test the molecular mechanisms by which Foxn1 maintains TEC homeostasis and function.