Since the recognition that asthma is genetically determined, enormous progress has been made to understand which genes determine disease development in interaction with each other and/or with different environmental factors. This is the result of rapid developments in techniques for genotyping and statistical analyses. However, we are only at the beginning of understanding the complex nature of asthma. Follow-up of both clinical and environmental measures in a standardized way in numerous prospective cohorts is needed to establish which sets of genes and environmental factors determine the different phenotypes of asthma. This includes differences between sex, asthma severity, asthma remission, and asthma progression. Investigation of genetic profiling and gene expression profiling may further help to better understand the underlying pathophysiologic mechanisms, taking into account the insight in the biology currently present. Only by collaborative efforts of many groups of researchers that join forces in DNA analyses will it be possible to help to develop preventive strategies for asthma.