Genetic studies have been hampered by the lack of a gold standard to diagnose asthma. The complex nature of asthma makes it more difficult to identify asthma genes. Therefore, approaches to define phenotypes, which have been successful in other genetically complex diseases, may be applied to define asthma in genetic studies. These approaches include narrowing of the disease definition and use of intermediate phenotypes of asthma. Future studies are required to apply these approaches in genetic studies of asthma and, most likely, this will facilitate the search for genes for asthma.