Following a brief review of recent advances in the field of immunology, the concept of cell-mediated immunity is discussed in relation to new findings regarding the induction and function of sensitized T cells. Cytotoxic T lymphocytes are taken as an example of effector cells involved in cell-mediated cytotoxicity. Due to the development of in vitro model systems, it is now feasible to study the various steps leading to the generation of cytotoxic lymphocytes as well as the mechanism of their activity. For comparative purposes, other models of cell-mediated cytotoxicity are briefly discussed.