This concept, which has been around for some twenty years, designates the ensemble of knowledge and cognitive gains gathered through the whole of the activities carried out over a lifetime (school education, leisure activities, social interactions; etc.). These activities thus develop a kind of reserve which compensate for the natural ageing of cognition or the deterioration of mental faculties caused by certain diseases such as Alzheimer’s disease or multiple sclerosis. Two hypotheses exist to explain this cognitive reserve mechanism. A quantitative hypothesis, which strives to show that putting the brain to work through multiple activities allows the stimulation and an increase in the number of neurones, glial cells and synaptic connections. In other words, the ensemble of activities carried out allows the ‘cerebral mass’ to be increased. The qualitative hypothesis for its part explains how a person confronted by a difficulty or a problem will naturally and sometimes unconsciously find a different oblique path to solve the problem. For example an old person who can no longer find a word (‘having a word on the tip of one’s tongue’) and who has a low cognitive reserve will be more easily blocked in forming a sentence. On the other hand a person who has a higher cognitive reserve will find a circumlocution or a synonym to overcome the difficulty, and which will in no way hinder their ability to communicate. In this case it is thus not the cerebral mass which influences intellectual capacities, but rather the ability to make use of other cerebral networks which allow the difficulty encountered to be compensated.