Members of the genus Aspergillus are opportunistic pathogens for cold- and warm-blooded animals. They are associated with an impressive spectrum of diseases in humans, ranging from benign colonization of the lung to life-threatening diseases such as invasive aspergillosis or allergic bronchopulmonary aspergillosis (ABPA). Aspergillus fumigatus is the etiological agent identified in most of the Aspergillus-related human diseases and is therefore of particular clinical relevance. A major problem in the diagnosis of A. fumigatus-related complications arises from the lack of standardized fungal extracts. The allergenicity of commercial A. fumigatus extracts depends on many factors including the strain used, growth conditions, harvesting and extraction procedures hampering the development of diagnostic reagents suitable for the discrimination between A. fumigatus-related diseases. Molecular DNA/RNA technologies and biotechnological procedures allow cloning, characterization and production of large amounts of single, highly pure allergens. The development of phage surface display technologies for cloning cDNAs has speeded up the isolation of candidate cDNAs encoding allergens. Screening of an A. fumigatus cDNA library displayed on the surface of phage M13 allowed characterization and production of a panel of allergens identified as proteins with known biological function or as allergens without sequence homology to known proteins. They belong to two functional categories: secreted and cytoplasmic proteins. Secreted allergens are recognized by serum IgE of A. fumigatus-sensitized individuals with or without ABPA, whereas nonsecreted allergens are exclusively recognized by serum IgE of ABPA patients. The dissection of IgE-mediated immune responses to single A. fumigatus allergens allows a discrimination between ABPA and A. fumigatus sensitization with high specificity and sensitivity demonstrating the power of recombinant allergens for the differential diagnosis of A. fumigatus-related pulmonary complications.