Several studies have indicated that the incidence and prevalence of allergic diseases are increasing. Such data should not be regarded as mere statistical curiosities, but should be analysed to provide information on the factors contributing to allergy and its changing epidemiology. Extensive evidence has been accumulated that allergic diseases are polyfactorial. Data reported in the literature and particularly twin studies have also suggested polyfactorial control of individual allergy variables, such as total serum levels IgE and IgG4, specificity of antibody response, mediator release from inflammatory cells and target organ response. Markers of genetic susceptibility may identify individuals at risk for allergy, while identification of the environmental factors influencing the phenotypic expression of allergy can be useful in evaluating the cost-benefit ratio of taking preventive measures in such individuals.