The thymus is an evolutionarily ancient primary lymphoid organ common to all vertebrates in which T cell development takes place. Failing thymus function is associated with immunodeficiency and/or autoimmunity. In this volume, leading experts provide a comprehensive overview of recent advances in thymopoiesis research. The chapters cover the development of the thymic epithelial microenvironment, address the formation of a diverse and self-tolerant repertoire of T cell receptors as the basis for cellular immunity, discuss the mechanisms by which progenitor cells colonize the thymus and detail the molecular basis for T lineage decisions. The reviews illustrate the important role of the multifaceted process of thymopoiesis for adaptive immunity.