Alzheimer's disease is increasingly seen as an heterogeneous disorder with a variety of molecular pathologies converging on a final common pathway of abnormal amyloid deposition and tau phosphorylation. These result in the appearance of the senile plaques and neurofibrillary tangles and in the subsequent development of a cortical dementia with a prominent memory deficit, reflecting the regional distribution of pathology. Age and mode of onset, additional neurological features and family history have all been used as a basis for classification. A family history has proved most robust with the identification of three genetic loci associated with autosomal dominant familial Alzheimer's disease (FAD). Genetically defined pedigrees are important for exploring the relationships between specific molecular pathology and clinical phenotype and, by following at risk individuals, identifying the earliest features.