Allergen exposure plays a major role in the development of sensitization, inflammation and symptomatic asthma. The development of new means of detecting allergens has enabled the evaluation of the effect of a reduction in allergen exposure to be better assessed. In order to prevent allergen exposure it is necessary to determine the allergen sources and reservoirs to which avoidance measures should be directed. The allergen sources and reservoirs of mite, cat, dog, cockroach, rodent, mould and pollen allergens are briefly reviewed. The relationships between allergen exposure, sensitization, nonspecific bronchial hyperreactivity and asthma symptoms are discussed. Recognition of the risks, environmental control, and reduction in allergen loads should be among the objectives of asthma management.