The prevalence of asthma in a farming environment is significant (from 3-7.7% according to various studies). The clinical picture has not been specified. The responsible agents are multiple and polysensitization frequent. Numerous substances of vegetable origin may be a cause, pollens, moulds, cereals, oil and protein producing plants, fibrous textiles, diverse plants and wood, etc. Amongst substances of animal origin the most frequent allergens incriminated are: allergens from mammals (horses, cattle, pigs), allergens from chickens and birds, arthropod allergens, insect allergens. Amongst chemical products those which are at risk to the respiratory system include insecticides, herbicides, fungicides, antibiotics and antiparasites. The diagnosis of the occupational character of asthma is sometimes difficult because of other causes of lung diseases in farm workers. The aetiological diagnosis is achieved by the clinical history, cutaneous tests, a level of specific serum IgE, and bronchial provocation tests which contrary to asthma in the industrial environment are not fundamental to the diagnosis except where chemical products are suspected. The mechanisms are sometimes intricate (IgE dependant mechanisms, non-specific liberation of histamine, activation of complement, intervention of endotoxins and pharmacological mechanisms...). Prevention may be carried out on behalf of the victim (for example wearing filtration masks in dusty work) and also carry out various methods to reduce the level of exposure to the allergen.