Primary immunodeficiency diseases consist of a group of more than 100 inherited conditions, mostly monogenic, predisposing individuals to different sets of infections, allergy, autoimmunity and cancer. Primary immunodeficiencies therefore represent exquisite models of various immunopathological settings. The identification of the associated genes, 100 so far, has generated a plethora of information about the immune system and spurred the analysis of many aspects of the development, function and regulation of both innate and adaptive immunity. These findings can potentially contribute to improved care of affected individuals by providing new diagnostic and/or therapeutic tools.