Bilingualism is the ability to use two languages for communication purposes. Although it was once considered a disadvantage of bilingualism, today it is known that bilingualism has a number of advantages (cognitive, cultural, communicative...). Compared to monolingual speakers, bilingual speakers have a more complex mental lexicon in which two languages are stored. According to numerous authors, there are two basic groups of bilingual mental lexicon models: hierarchical and connectionist. In a conversation with one language speaker, the bilingual speaker needs to learn to suppress the other language by using more complex cognitive abilities of language control. However, in communication, there is often interference from another language and a phenomenon called code-switching. Code-switching is simply defined as switching between two languages during an interaction. In the early phase of bilingual development, this phenomenon is not established, so we say that language mixing occurs in children, because the grammatical structure of their utterances is not correct. Adult speakers often switch codes for various reasons (poor vocabulary in one language, group solidarity, resolving disagreements...). In code switching, there is a language base that controls the production and a guest language from which the codes are switched. Depending on the situation, code switching is more or less present. Various authors state that there are clear rules about the code-switching process, but in the spontaneous speech of bilingual speakers, this process often violates all rules and predictions.