In many medical textbooks allergens are still listed as one of the "trigger factors" for asthma, with the implication that irritability of the lungs is an underlying physiologic property of the lungs. Over the last few years it has become clear that asthma is an inflammatory disease. In addition, it has been shown that changes in allergen exposure can lead to changes in bronchial reactivity. These results lead to the view that continued allergen exposure of the lungs of allergic individuals with resulting "inflammation" is an important primary cause of asthma. In parallel with this has come a progressive increase in awareness and understanding of the importance of house dust allergens. In many parts of the world, the dust mite dominates house dust allergy and appears to be responsible for up to 85% of asthma. In the United States three indoor allergens appear to play an equal role: dust mites, cats, and cockroaches. In the future, management of asthma will increasingly focus on allergen avoidance both as a method of treating symptomatic allergic patients and also as an approach to controlling the development of a disease which currently has increasing morbidity and mortality.