The molecular characterization of allergens with recombinant DNA techniques allowed cDNA sequences to be obtained and, hence, information regarding the primary structure of allergens. It is now possible to express well-defined recombinant allergens in heterologous expression systems and to obtain large amounts of highly pure recombinant allergens for the improvement of current diagnosis and therapy of type I allergic diseases. Due to extensive cross-reactivities and structural similarities of the relevant allergens it is possible to define a limited number of allergens, which is a prerequisite for allergen-specific therapeutic concepts. Specific concepts of active immunotherapy and strategies of passive therapeutic interference based on recombinant techniques are discussed.