This article highlights the fact that learning a third language in school contexts is a common phenomenon all over the world and poses several questions specifically related to the characteristics of third language acquisition. It also considers the relationship between third language acquisition and the research traditions of bilingualism and second language acquisition. Third language acquisition in school contexts and trilingual education are regarded as multidisciplinary phenomena associated with the sociolinguistic context in which they take place, the psycholinguistic processes involved in acquiring more than two languages, the linguistic characteristics of the languages involved and the pedagogical aspects of teaching and learning several languages.