This article is a literature review focusing on the underlying concepts of human genetics related to asthma and allergies. Asthma is classified as a complex genetic disorder with genetic susceptibility and an appropriate environmental stimulus necessary for the expression of disease. Intermediate phenotypes of asthma, which are used to study the disease, include the total IgE levels, the specific immune response, and end organ response. Because gene products cause the inflammation in asthma and allergies, the molecular cell biology of these events is reviewed. Although no one gene is implicated in causing asthma, candidate genes involved in the multiple phenotypic expressions of asthma are described.