Our understanding of the function of natural killer (NK) cells has dramatically changed in recent years. The discovery of NK receptors specific for MHC class I molecules, and the study of the role of co-stimulatory and adhesion molecules have led to an understanding of how NK cells recognize tumor and virally infected cells that have lost expression of MHC class I molecules or have altered distribution of normal cell surface molecules. Such recognition events lead to intracellular signals which can be either stimulatory or inhibitory. This book provides an insight into how NK cells develop, how they learn to distinguish altered cells from normal cells, and into their biological role in controlling infections and tumors.