T cell development occurs in the thymus in both miceMouse and humans. Upon entry into the thymus, bone marrowBone marrow-derived blood-borne progenitors receive instructive signals, including Notch signalingNotch signaling, to eliminate their potential to develop into alternative immune lineages while committing to the T cell fate. Upon T-lineage commitment, developing T cells receive further instructional cues to generate different T cell sublineages, which together possess diverse immunological functions to provide host immunity. Over the years, numerous studies have contributed to a greater understanding of key thymic signals that govern T cell differentiation and subset generation. Here, we review these critical signaling factors that govern the different stages of both mouseMouse and human T cell development, while also focusing on the transcriptional changes that mediate T cell identity and diversity.