Asthma is a complex disease with a phenotype that has been clinically difficult to define. Associated phenotypes including bronchial hyperresponsiveness and atopy have provided useful objective alternatives in genetic and epidemiologic studies. Although asthma genes have not yet been identified, much progress has been made toward this goal. Genetic studies indicate that multiple genes are involved in the pathogenesis of this disease, and chromosomal regions likely to harbor asthma susceptibility genes have been replicated in several studies. Environmental factors, including smoking, diet, and viral respiratory infections, have also been implicated in the etiology of asthma. Directly linking these exposures as causes of asthma, however, has also proved difficult. Furthermore, interaction between susceptibility genes and environmental factors is probable and is a challenge currently being pursued by investigators worldwide. Understanding the fundamental gene-environmental interactions in the development of asthma should lead to earlier identification of susceptible individuals and more effective approaches for disease prevention.