Multilingualism is the use of more than one language, either by an individual speaker or by a group of speakers. It is believed that multilingual speakers outnumber monolingual speakers in the world's population. More than half of all Europeans claim to speak at least one language other than their mother tongue; but many read and write in one language. Multilingualism is advantageous for people wanting to participate in trade, globalization and cultural openness. Owing to the ease of access to information facilitated by the Internet, individuals' exposure to multiple languages has become increasingly possible. People who speak several languages are also called polyglots.

Multilingual speakers have acquired and maintained at least one language during childhood, the so-called first language (L1). The first language (sometimes also referred to as the mother tongue) is usually acquired without formal education, by mechanisms about which scholars disagree. Children acquiring two languages natively from these early years are called simultaneous bilinguals. It is common for young simultaneous bilinguals to be more proficient in one language than the other.

People who speak more than one language have been reported to be more adept at language learning compared to monolinguals.

Multilingualism in computing can be considered part of a continuum between internationalization and localization. Due to the status of English in computing, software development nearly always uses it (but not in the case of non-English-based programming languages). Some commercial software is initially available in an English version, and multilingual versions, if any, may be produced as alternative options based on the English original.