Culture, education and of other forms of acquired capacities act on individual differences in skill to shape how individuals perform cognitive tasks such as attentional and executive control. Of interest, the use of more than one language (bilingualism) also appears to be a factor that shapes individual performance on tests of cognitive functioning. Indeed, researchers have shown that a bilingual can have better attention and executive control capacities than monolingual speakers and this is argued to be due the ability to inhibit one language while using another. Beyond behavioral differences, bilingualism seems to affect brain structure as well. Recent evidence also shows bilinguals develop more gray matter in crucial brain areas responsible for executive control, hence, providing a neurological basis for why bilinguals outperform monolinguals on many attentional control tasks. It has been postulated that this cognitive advantage offers protection to bilinguals against cognitive decline in aging. Bilingualism affords a cognitive reserve in the form of a set of skills that allows some people to cope with cognitive decline such as mild cognitive decline or Alzheimer's disease better than others. The primary aim of the studies here performed was to investigate if and how the bilingual brain becomes more resistant to cognitive decline. Three combined comparative behavioral and structural neuroimaging studies were carried out in bilingual and monolingual seniors. The overall results show a rather interesting pattern of findings that may be summarized as follows: if well matched for demographic and behavioral variables such as age, socio-economic status, education, and global cognitive functioning, bilinguals have generally increased gray matter densities as compared to monolinguals in those brain areas that are known to be more affected by physiological aging such as the orbitofrontal cortex, the temporal poles and parietal lobules, and in areas involved in cognitive control such as the prefrontal cortex and the anterior cingulate cortex. Increased gray matter in these latter areas also correlates with the superior performance of bilinguals on executive control tasks. Interestingly, in order to keep such a neural benefit (i.e. increased gray matter density) the degree of proficiency of the second language has to be relatively high and bilinguals have to be constantly exposed to their second language. Finally, specifically for the aging population, age of second language acquisition has no major role in determining putative neural differences. Any putative neural differences between bilingual speakers are determined by factors such as the degree of proficiency and exposure to a second language. In conclusion, as thoroughly investigated here, bilingualism represents a neural reserve for healthy aging. However, the benefits are most prominent when second language proficiency and exposure are kept high. / published_or_final_version / Education / Doctoral / Doctor of Philosophy