Research in disease aetiology has shifted towards investigating genetic causes, powered by the human genome project.1 2 Successful identification of genes for monogenic disease has led to interest in investigating the genetic component of diseases that are often termed complex—that is, they are known to aggregate in families but do not segregate in a mendelian fashion. Genetic epidemiology has permitted identification of genes affecting people's susceptibility to disease, although progress has been much slower than many people expected. While the role of genetic factors in diseases such as hypertension, asthma, and depression is being intensively studied, family studies and the large geographical and temporal variation in the occurrence of many diseases indicate a major role of the environment. Thus, it is necessary to consider findings about susceptibility genes in the context of a population and evaluate the role of genetic factors in relation to other aetiological factors.This article discusses some approaches used to resolve the genetic architecture of disease and to study the relation of genes to environmental factors in the population.