Sensitivity to fungi is a major risk factor for the development of asthma. However, the prevalence of fungal sensitivity in asthma is not completely understood. Nonetheless upward of 80% of asthmatic patients may be sensitized to one or more fungi. Fungal exposure occurs primarily from outdoors sources, but can occur in the indoor environment as well. Assessment of fungal exposure requires a multifaceted approach including measurement of airborne spores and culture techniques to identify the relevant organisms. Preventing intrusion of outdoor fungal spores into the indoor environment may be helpful in reducing allergic symptoms. Methods to abate indoor fungal growth include reduction of indoor humidity and removal of water sources. Patients with fungal sensitivity should be advised to avoid exposure as much as possible. For patients who have failed to respond to environmental control measures and appropriate medications, it may be reasonable to consider specific immunotherapy. The application of molecular biology techniques to the study of allergens has enhanced the researcher's ability to produce Alternaria allergens in quantity, to determine their biological relevance, as well as to evaluate mechanisms of Alternaria sensitivity. We look forward to new developments and improved treatments through modulation of the immune response with molecularly produced and well characterized fungal allergy.