IMMUNOLOGY, the third volume in the four volume set, THE MOUSE IN BIOMEDICAL RESEARCH, is a completely new addition to this series, dedicated to mouse immunology. It is based on the vast body of knowledge which has made the mouse the model of choice when studying immunity in man. Arguably more is known about the immune system in mice than any other species except man. In large part this is due to the power of genetic engineering to delineate molecular mechanisms. In this volume we present an Overview to mouse immunology, including both the innate and adaptive immune systems, followed by 15 chapters, each dealing with a specific area of immunology in the mouse. These chapters illustrate the power of genetic engineering in dissecting each component of the immune response from the development of lymphoid tissues to signal transduction pathways in activated cells.