Asthma and atopy are due to the interaction between genetic and environmental factors. The genetic basis of asthma and atopy is becoming more certain, with the promise of improvements in the diagnosis and treatment of these disorders. A combination of candidate gene and positional cloning studies has already identified several genes which influence allergic disease. These genes predispose in general to atopy, or influence the specific IgE response to individual allergens, or enhance inflammation independently of atopic mechanisms. In addition to these known factors, the chromosomal localisation of other putative genes is becoming clear.