T cell development in birds and mammals is remarkably similar. One hallmark of these similarities is the division of T cells into two sublineages characterized by different T cell receptor (TCR) molecules - the γδ and αβ TCRs. These two sublineages differ in several ways besides the molecular form of the TCR, such as the mechanisms involved in TCR repertoire selection and their pattern of tissue localization. The extensive similarity between the avian and mammalian γδ T cells suggest that the separation of these two lineages is relatively ancient evolutionarily. The advantages of the avian system as a developmental model have allowed information to be obtained on the γδ T cells which may be of general relevance. © 1991 by W.B. Saunders Company.