The main aim of this volume is to provide an updated overview of the genetic basis of a group of complex disorders that are considered rare individually, but show a relatively high prevalence on the whole in developed countries: the rare autoimmune diseases. These are chronic disorders that involve long and expensive treatments with considerable side effects that may dramatically reduce the quality of life of affected people. Therefore, shedding light into their pathologic mechanisms is a major concern given their growing social and economic awareness and impact.  General sections include different chapters on single entities as systemic lupus and sclerosis, Behçet s disease, Sjogren s syndrome and polymiositis, and sections devoted to autoimmune vasculitis, hepatic autoimmune conditions, as well as those affecting the nervous system, and a concluding chapter in which the shared predisposition amongst all diseases is discussed.