T cells play an important role in regulating the immune response to virtually all antigens. Central to the functioning of these lymphocytes is their capacity to recognize and respond to antigen. Although the T cell receptor has yet to be defined in precise terms, considerable information has accrued concerning its structure and function. This article discusses certain aspects of T cell recognition which are required for a general understanding of the physiology of an immune response and which may eventually permit selective modulation of a specific immune response in various patients including those with neoplastic disorders.