Among the different factor (allergens, pollutants, infections) related to the genesis of asthma, only the allergens are capable of generating the type of inflammation that is characteristic to the illness, thus being able to take into account the etiological factors of the said illness. The role of the aerial allergens in the development and seriousness of asthma has been shown, the patients improvement if he is separated from the allergen (high altitudes for people allergic to mites). Even with important methodological difficulties, in controlled clinical tests we have been able to establish the effectiveness of the antiallergenic measures in the handling of asthma, if they are effective in the reduction of the allergenic charge. Avoiding allergens for primary and secondary prevention has a logical and clinical basis but tests on a longer term are necessary and using a larger number of patients before making final conclusions. With respect to the tertiary prevention, several controlled clinical tests have demonstrated the efficiency in the reduction of the allergenic charge and the effectiveness in handling asthma. In addition to eliminating or reducing the contact with pets and pollen when indicated, we should also try to reduce the charge of the mites (an etiological factor in more than 80% of asthma in almost all the patients). It is recommended to use impermeable or semi-permeable covers on mattresses and pillows, to avoid upholstered furniture (at least in the patients bedroom), carpets and rugs and to wash bed linen with hot water, to air the room daily and to avoid exposure to tobacco smoke. There are doubts regarding the efficiency and the safety, on a long term basis, of insecticides, and their use is not recommended according to the latest advice on the handling of asthma.