There is evidence that, in some areas of the Western world, the prevalence of asthma may have plateaued. The environmental factors causally driving the temporal changes remain largely unknown. Therefore, there are few truly justified recommendations for the prevention of asthma. Avoidance of passive and active exposure to smoke is warranted for many other health reasons and also for asthma prevention, because the epidemiologic evidence strongly suggests a causal link between exposure to smoke and the onset of asthma. By contrast, many other proposed avoidance strategies, such as reducing allergen levels, implementing changes in diet, withholding vaccinations or treatment with antibiotic and antipyretic agents, administering probiotics, or even exposing children to pets early in life, are either ineffective or unverified as primary prevention measures. Our view, however, is that the rapid progress in the identification of protective microbial substances has great potential for the development of prevention strategies to combat the asthma epidemic.