As individuals age, their ability to respond to and clear pathogens and to control unwanted immune reactions declines, leading to a greater incidence of certain infectious diseases, autoimmunity and general immune dysfunctions. Most remarkably, the efficacy of vaccines is frequently decreased in elderly persons. Therefore, age-associated dysfunctions of the humoral and cellular immune responses have a strong clinical impact. Improving our understanding of the aged immune system is crucial in developing effective prevention and treatment programs that will facilitate healthy aging and improve the quality of life of the elderly population. The aim of this volume is to summarize current knowledge on the cellular and molecular aspects of the aging immune system, with an emphasis on infectious diseases and new therapeutic approaches.