An immensely complex network of interactions between multiple cell types and cytokines regulates the immune system. Ligand binding to antigen-specific, Fc, or cytokine receptors initiates differentiation, activation and proliferation of multiple immune system cell lineages. These activating pathways are counteracted by inhibitory receptors. In the past two decades, considerable work on mechanistic and functional details of such intracellular signaling pathways has led to the realization that an excess of stimulatory/positive signals as well as a deficiency in inhibitory/negative signals can both result in a hyperactive immune system leading to autoimmunity. The chapters included in this volume represent but a few examples of the close link between aberrant signaling pathways and autoimmune diseases. They cover a variety of cells (T, B and myeloid/monocytic cells), receptors (for antigen, Fc and cytokines) and intracellular signaling molecules (kinases, phosphatases, adapters and transcription factors) in the immune system. This book brings together clinical and experimental aspects of autoimmune disease and the fundamental science of intracellular signaling pathways. Therefore, it should be of interest to clinical investigators of autoimmune diseases as well as to basic immunologists and cell biologists interested in the molecular basis of signal transduction in the immune system.