The "Brain Reserve" hypothesis was raised after the analysis of the results of the first longitudinal surveys comparing in nuns the neuropsychological aging process and the post mortem brain pathological findings. Numerous discordances were discovered: a) Persons with high education, high IQ and complex working responsibilities could benefit of a healthy cognition until death while their neuro-pathological brain findings were concordant with the International pathological criteria of Alzheimer disease and b) Moreover the same persons with a high brain reserve suffering from Alzheimer Disease have late, atypical symptoms and signs of the disease explaining a delayed diagnosis. In these patients the progress of the disease is always quick and severe.