the idea and willing to participate. We thought it important to broaden the scope of the book to include historical, molecular, cell biological, and clinical aspects of cell-mediated cytotoxicity. To our knowledge this is the first book on cell-mediated cytotoxicity with such a broad scope. Historically, studies on cellular cytotoxicity were part of cellular immunology from its origin. One development of tremendous import was the advent of the 51 Cr assay, which allowed this arm of the immune response to be measured easily and quantitatively. Thus, a readout of this effector pathway is available within a few hours; other immune effector functions can take days or even longer to assay, and the assays are often less quantitative.