Individual differences in multilinguals are considered as group-specific behavior and processing. For this review paper, we selected two lines of research to look into more specifically. In the first research project, two groups of multilinguals (early and late trilinguals) are compared on a sentence processing task using fMRI. Findings showed the specific differences in processing due to age of onset. The second project compared two groups of healthy bilinguals differing in their susceptibility to switch unintentionally between two languages. A variety of tasks and different methods (including psycholinguistics and electroencephalography) allowed isolating the key factor distinguishing both groups: cognitive control (assessed by executive functions tasks). The non-switcher group was more efficient, faster and more correct, whereas the switchers showed difficulties of language control and cognitive control, in particular being more distracted by irrelevant information.