While allergic diseases comprise the most common chronic disease in childhood, obesity is the most prevalent nutritional disorder among children throughout the world. In Europe, an estimated 20% of children and adolescents are overweight with one-third of these being considered obese. Moreover, escalation of these problems is expected in the future, since the velocity of propagation is highest in children. Although genetic factors can determine the propensity of an individual to become allergic or obese, these unlikely explain the recent and progressive worldwide increases in incidence. Rather, it would appear that the environmental changes more directly shape the risk during a critical period of life when the scene is set for the consolidation of the immune responder type. Prenatal environmental exposures may alter gene expression via epigenetic mechanisms, heritable changes in gene expression occurring without alterations in the DNA sequences.