An increase in the prevalence of atopic diseases in industrialized countries has been noted for several years. This has resulted in greater interest in analyzing possible factors that may influence the development of allergic disease, particularly during the perinatal and early infancy periods. In this paper we will discuss various factors during late pregnancy and early infancy that may influence the development of allergic disease later in infancy and childhood (Table 1). The effect of the various external factors to be discussed should always be considered in relation to the genetic constitution of the individual. Several recent studies have shown that the impact of environmental risk factors is quite different in individuals with and without the genetically determined propensity to become sensitized (Fig. 1).