Allergen-specific T cells play an important role in the pathophysiology of atopic allergies. Recently, cDNAs that encode many important allergens have been cloned and their amino acid sequences deduced, thus allowing the elucidation of the epitope-specificity of allergen-specific T cells. Here, Joost van Neerven and colleagues discuss the results of these studies, and the implications for the development of efficient strategies for specific immunotherapy.