We have known about lymphocytes that kill other cells since 1960. Since then we have uncovered five different subsets of these killer lymphocytes. In this book we summarize the extensive literature on these cells, gathered largely in vitro, which has shown us exactly how they kill other cells. But do killer lymphocytes actually use these mechanisms in vivo? The possible involvement of these cells in transplant rejection, control of intracellular parasites, cancer, autoimmunity and homeostatic immune regulation is analyzed in detail, yielding some very surprising results.