This WikiChapter summarized and gave an overview on how the acquisition of a second language would affect an individual’s cognitive and language aspects. In general, research show evidence of positive effects in the cognitive aspects, proposing that having knowledge of more than one language aids individuals in concept categorisation, conflict resolution and decision bias tasks, improving and altering their thought processes. Furthermore, in the language aspect, although there is a possibility of subtractive bilingualism occurring, additive bilingualism can result if one constantly uses and speaks their first language.  In addition, there have been extensive research done on language transfer from one’s L1 to L2 – using knowledge of L1 and applying them to better acquire the second language. Interestingly, it was also found that language transfer occurs in the other direction from L2 to L1, bringing about positive effects on one’s first language skills.