The field of allergy has markedly benefited of biotechnology progresses leading to the identification and cloning of numerous common allergens. This progresses have contributed to better understand the physiopathology of some complex allergic manifestations such as the oral allergy syndrome. Furthermore, the sensitivity of in vitro diagnostic tests appears to be improved by the use of recombinant allergens; in vivo, they are also able to trigger an immediate hypersensitivity reaction. In a near future, recombinant allergens may advantageously replace classical allergen extracts. Finally, the characterization of IgE binding sites on recombinant allergens, as well as T cell epitopes, open the possibility to use recombinant allergens in various immunotherapeutical approaches based on the administration of allergen peptides, allergen fragments, allergen isoforms or mutated allergens. Since those allergen preparations have a low affinity for IgE, they may substitute for classical allergen extracts in conventional immunotherapy, and improve the security of the treatment.