Progressive changes in American housing and life styles have been associated with increased prevalence of allergen sensitization and asthma. Not only have there been large increases in the proportion of time spent indoors, but many of the changes made in houses are likely to increase exposure to indoor allergens. Thus, higher mean indoor temperatures, reduced ventilation, cool wash detergents, and the widespread use of carpeting are all changes that could have increased the levels of allergens in American homes. Over the past 15 years, dust mites, cockroaches, and cats have been identified as major sources of indoor allergens. The combination of exposure and sensitization to one of these allergens is significantly associated with acute asthma. Furthermore, clinical studies have shown a direct quantitative correlation between dust mite allergen exposure and the prevalence of both sensitization and asthma. New evidence suggests that reductions in allergen exposure may improve asthma symptoms, leading to decreased inflammation and bronchial hyperreactivity. Furthermore, as understanding of sources of allergens increases, the protocols for decreasing exposure become better defined and relatively easy to implement.