Understanding of the differentiation and maturation of T cells has been greatly enhanced by several technical developments. Certainly, the most important of these has been the rapid expansion in the availability of monoclonal antibodies to identify surface markers on T cells. Utilization of these antibodies has permitted the description of the developmental sequence of T cells both within the thymus and peripherally. Equally impressive have been the rapid advances in the understanding of the cellular events involved in antigen activation of T cells. These events are MHC restricted. The biochemical description of the class I and class II molecules that are involved in antigen presentation and the biochemical description of the T-cell receptor have provided a framework that should allow a more definitive picture of these specificity-determining molecular structures to emerge in the coming years. Several soluble growth and maturation factors (IL-1, IL-2, and BCGF), which are involved in the generation of cellular immune responses, have also been integrated into the above framework. Finally, ongoing studies on products of activated T lymphocytes (lymphokines) are providing clearer insights into the mechanisms by which immune T cells influence other cells. An additional benefit from the study of T cell development has been a more biochemically based classification of T-cell malignancies. The powerful tools of molecular genetics are beginning to have an impact on research in T-cell differentiation and maturation, and will likely have an ever-growing influence on research into the phenomena surrounding the activities of these immune cells.