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What is genetic epidemiology? Epidemiology is usually defined as “the study of the distribution, determinants [and control] of health-related states and events in populations”.1 By contrast, genetic epidemiology means different things to different people.2–7 We regard it as a discipline closely allied to traditional epidemiology that focuses on the familial, and in particular genetic, determinants of disease and the joint effects of genes and non-genetic determinants. Crucially, appropriate account is taken of the biology that underlies the action of genes and the known mechanisms of inheritance. The word “appropriate ” is crucial because the manner in which biology is taken into account varies from setting to setting and depends on the genetic informatio