The ability of an organism to combat infection by foreign particles and microbial pathogens is essential for its survival and evolutionary success. Such efforts at immunity can take two forms. A considerable number of works have been published which focus on the central role of antigen recognition and antibody structure and function in the host response to infection. This volume, however, discusses the recent shift in focus towards the "natural" or "innate" immune system which consists of various cell types and factors. These cells and factors can take part in immune responses without prior sensitization, and have important modulatory effects on later, specific responses. This work reviews the biology and function of the natural killer cell, covering such topics as the molecular basis of natural killer cell function and its role in viral infection, tumor biology, and transplantation. Graduate students and researchers in immunology, cell biology, and medicine will find this work a valuable resource on current research in this exciting field.