Bilingual people are those who speak two languages. However, bilingual people can master a language to different levels, begin learning another language at different ages, and are exposed to each of these languages to a different level. The brain distinguishes between two languages and "knows" when each was learned, and these two languages occupy different neural circuits in the brain. There is a critical period in a child's development for learning a second language. When the critical period begins and ends has been debated for the past few decades. In bilingualism, in addition to the basic parts of the brain responsible for hearing and speech, it is important to understand the role of other parts of the brain involved in switching from one language to another and inhibiting the language that we do not want to use at that point. Also, in the development of bilingualism, the importance of brain plasticity and its constant changes in learning and using both languages must not be neglected. The most significant studies of changes in the brain are those in gray and white matter. Despite the great advantages of bilingualism, some scholars also investigate the disadvantages of knowing two or more languages. Due to the large variability of the data obtained by different approaches to experiments, it is very important to discuss the impact of these experiments on the results and possible conclusions. Finally, this review explains the connection between bilingualism and aphasia and the way in which the integrity of the first and second languages is preserved.