Dementia of the Alzheimer type (DTA) is the most common form of adult-onset dementia. The clinical features of the disease include progressive memory defect, intellectual impairment and behavioral disturbances. The number of patients suffering from DTA is increasing dramatically, due to the growing proportion of old people. DTA therefore is a major public health problem. Brain lesions include several histopathological changes (mostly in the cerebral cortex and hippocampus): neurofibrillary tangles, senile plaques, granulo-vacuolar degeneration, Hirano bodies, angiopathy, loss of nerve cells. Postmortem brain examination reveals characteristic neurochemical deficits. The most consistent reduction in neurotransmitter-synthesizing enzyme observed so far is a reduction in choline acetyltransferase activity, suggesting a cholinergic deficit. However, other deficits involving noradrenaline and somatostatin have also been reported. Several hypotheses (genetic, viral, toxic, immunologic, metabolic) have been put forward in order to explain DTA. The relationship between these hypotheses and the neurochemical deficits is still unclear, but the existence of characteristic neurotransmitter-related deficits allows specific therapeutic trials.