The most important morphological findings in dementia of Alzheimer type (DAT) are Alzheimer neurofibrillary tangles, senile plaques, amyloid angiopathy, granulovacuolar degeneration and Hirano bodies. The morphological and immunocytochemical findings in these changes are described, in particular those related to the pathological cytoskeleton. Their possible relationship to the disturbed synthesis of neurotransmitters recently demonstrated is considered, and their relevance for the clinical syndrome of dementia is discussed. Hypothetical etiologies (ageing per se, genetics, infection and chronic intoxication) are briefly mentioned.