We have known about the existence of killer lymphocytes since 1960, when they were discovered in connection with transplant rejection in vivo. Since then we have uncovered at least five subsets of lymphocytes that can kill other cells in vitro, establishing the study of cell-mediated cytotoxicity (CMC) as a major field of immunological inquiry. Berke and Clark summarize the extensive literature based on the study of CMC in vitro. Several important questions about killer cells have now been answered, for example, how they go about destroying other cells. Research ultimately revealed at least three lytic mechanisms available to killer lymphocytes. But do killer cells actually use these mechanisms in vivo? The possible involvement of CMC in transplant rejection, control of intracellular parasites, cancer, autoimmunity, and immune homeostatic regulation is analyzed in detail, yielding some surprising findings, and outlining important questions that remain unanswered.