Alzheimer's disease comprises senile and presenile dementia. Aetiologically the disease is heterogeneous but has common clinical and pathological characteristics. Furthermore, in patients with a family history of dementia, its incidence is higher than in the general population. In families which inherit early-onset forms of the disease in an autosomal dominant fashion, several apparently pathogenic mutations were identified in three different genes. Dominant hereditary factors are not known for the much more common late-onset forms of Alzheimer's dementia. There are, however, genetic risk factors which contribute to the development of the disease. To date just one of these factors, the apolipoprotein E allele epsilon 4 has been definitively identified. Genetic testing can currently only be done for prognosis of the early onset but not the late onset forms of the disease.