Abstract
<jats:p>Hematopoiesis is a highly organized process that begins during embryonic life and is maintained throughout the lifespan. The transition from fetal hematopoietic sites to adult bone marrow establishes an efficient cellular production system. Differentiation of hematopoietic stem cells (HSCs) into the lymphoid lineage is essential for adaptive immunity and depends on a complex network of interactions among molecular signals, the hematopoietic niche, and subsequent maturation within the thymic niche. Understanding these mechanisms is not only of biological importance but also has diagnostic and therapeutic implications for multiple hematological and immunological diseases. This chapter describes the ontogeny of T lymphocytes, as well as their stages of commitment, selection, and immunological function.</jats:p>