The role of primary prevention of allergic diseases has been a matter ofdebate for the last 40 years. In order to shed some light into this issue, agroup of experts of the Section of Pediatrics EAACI critically reviewedthe existing literature on the subject. In this paper, the immunology ofthe fetus and newborn is reviewed as well as the post-natal developmentof the immune system. The influence of post-natal environment andbreastfeeding on tolerance induction and sensitization are examined.Allergic diseases result from a strong relationship between genetic andenvironmental factors. Sensitization to food allergens occurs in the firstyear of life and cow’s milk allergy is the first food allergy to appear inthe susceptible infants. Hypoallergenicity of food formulas to be used isa critical issue both for treatment of cow’s milk-allergic children and forprevention. Methods to document hypoallergenicity are discussed andevaluated in the preclinical and clinical steps.