Allergic diseases are most likely due to interactions between genetic and environmental factors. While many of the environmental components have been studied for years, only recently has significant progress been made in identifying the genes responsible for susceptibility or expression of these diseases. Genome-wide screens in various populations have identified the locations of susceptibility genes for asthma and atopy, as well as associated phenotypes such as bronchial hyperresponsiveness and increased total serum IgE levels. In addition, this positional cloning approach has led to the discovery of several genes for asthma or related phenotypes, which is extending our understanding of the pathophysiology of allergic diseases. As these genes are identified and characterized, the relationships of these genes to each other and the environment will become important areas of research. Understanding the basic interactions that lead to the development of allergy and asthma will lead to new therapeutic approaches that will be used to modify the development and clinical progression of these common disorders.