This chapter focusses on avian T cell development. A renewed interest in avian T cell development has emerged with the use of monoclonal and functional antibodies to elucidate T cell differentiation antigens and molecular and functional definitions of mammalian T cell receptors (TCRs). Thousands of avian species have been identified; the domestic chicken has served as the avian representative in most studies of the immune system. Chick-quail chimeras have also proved to be an especially informative model. The chapter focuses on information gained in studies of these model systems. Monoclonal antibodies are produced against a variety of functionally important molecules expressed on the surface of chicken T cells and most of these have well-defined mammalian counterparts. The chicken T cell receptors can be divided into three groups, each of which is recognized by a different monoclonal antibody. Direct evidence for thymus-directed chemotaxis of hematopoietic precursors is obtained by observing migration of cells toward the thymic epithelium in an in vitro model. Comparative analysis of avian T cell development and function reveals striking conservation of the major features defined for the mammalian T cell system. In both mammals and birds, the T cells utilize either γ/δTCR or α/βTCR together with a CD3 protein complex as signal-transducing receptors for antigen presented in the context of major histocompatibility complex restriction elements. © 1991, Elsevier Inc. All rights reserved.