Our motivation for putting together this book was the need for a single source reference that could be used as an introduction to cell-mediated cytotoxicity for newcomers to this field, such as students and fellows beginning work in our laboratories. At present no such book is available, and we felt that it would be useful as a teaching tool and as a way of conveying our enthusiasm about recent progress in the cytotoxicity field to our colleagues in allied areas. It was with some hesitation that we approached our colleagues with the proposal for this book, and we were pleased to find them very supportive of 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.