Complex diseases usually harbour hereditary factors linked with multiple susceptibility genes. The additive effects of genetic and environmental factors are responsible for the pathology. The impact of heredity has been demonstrated through family studies, but also, and mostly, through the study of adopted people and twins. Recently, genome wide association studies (GWAS) allowed the identification of many susceptibility genes for most complex diseases. However, a large part of the heritability is still missing, probably because of insufficient exploration of rare genetic variants and/or epigenetic factors. The ultimate goal of these genetic studies is the definition of an individual risk leading to specific preventive measures (model "predict and prevent"), but this purpose remains very remote for the majority of complex diseases.