Allergy is developing into one of the most prevalent diseases affecting individuals in the very early days of life. While the cause of this epidemic is still unclear, it appears that the westernized life style is playing an important role, which includes nutrition, possibly air pollution as well as hygienic conditions. While epidemiologic studies were able to narrow down these factors, basic research discovered novel mechanisms that control the organism´s tolerance against allergens. Particularly interesting is the role of microorganisms that colonize or infect a host and thereby cause damage and immunological activation followed by sensitization or exacerbation of already existing sensitizations. However at the same time microbial activation of the immune system can help to generate a protective immunity that prevents allergen sensitization. The current book is collecting these evidences and connects epidemiologic and clinical mechanistic knowledge. Only the synthesis of this knowledge will help to find solutions to the ongoing allergy epidemic in terms of public health activities, prevention and therapy.