Monoclonal antibodies (mabs) are powerful tools for the quantification, detection, and targeting of specific molecules. Allergen-specific mabs are important for the quantification of major allergens in allergen preparations used for allergen-specific immunotherapy and allergy diagnosis. Indeed, progress in the understanding of the mechanisms of the immunological responses underlying allergic disease would not have been possible without the use of mabs. Quantification assays are also important in the assessment of environmental allergen exposure and monitoring of avoidance procedures.Mabs against human IgE provide the basis for various test systems for the detection of specific and nonspecific IgE. Mabs raised against IgE or defined cytokines or cytokine receptors have potential as neutralizing reagents in vivo for the treatment of allergic diseases.Allergen-specific mabs are also valuable tools for the localization of allergens within their source material and the characterization of allergens derived from natural sources and by recombinant technologies. Furthermore they are often used for the isolation of allergens from complex extracts by affinity chromatography. The procedure described in this chapter has been used successfully to produce mabs against numerous allergens from house dust mites, insect venoms, cat, hens egg white, tree-, grass-, and herb pollens, and fungi, with the ultimate aim of obtaining matched antibody pairs to establish two-site binding assays for the quantification of major allergens. The method has also been used successfully to generate mabs against human IgE.