The relation of age associated changes of cognitive functions to those of dementia diseases is not well investigated for very old age. Because aging as well as dementia diseases are associated with cognitive deficits, this leads to differential diagnostic problems in very old age. Relevant with respect to this differentiation are on the one hand the concepts of cognitive ageing, the dementia syndrome and dementia diseases, and on the other hand empirical findings with respect to 1. the neuropsychological crossectional pattern, 2. the premorbid intelligence or adult intelligence level, 3. the speed of decline. ad 1. The speed of cognitive processes shows a considerable reduction in normal aging. However, the reduction of learning and orientation as well as pronounced word-finding problems seem to be characteristic of the development of a dementia syndrome. ad 2. The importance of the level of adult intelligence is demonstrated by the possibility that a very old person with low adult intelligence level is diagnosed as demented without suffering from one of the dementia diseases. In the opposite case of a slowly progressive dementia disease in a person with a superior level of adult intelligence a diagnosis of dementia according the standard criteria can be given only in an advanced stage of the disease. ad 3. The importance of the speed of decline of cognitive performance for the diagnosis of dementia is discussed (e.g. < 1 point of the MMSE vs > 3 MMSE points per year). An optimization of the time course criterion (change-sensitive tests and an empirically determined cut-off) could improve the early dementia diagnosis, which relies up to now mostly on crossectional features. The more precise assessment of the deterioration speed would be an opportunity to investigate the factors or processes which determine the deterioration speed, knowledge of which in turn would be a starting point for development of therapy.