This article introduces this special issue by declaring that the studies contained here build on the idea that multilinguals, in the sense of learners or speakers that have more than two languages in their linguistic repertoire, are different from bilinguals and monolinguals in various ways. Several authors in the area of third language acquisition (TLA) have referred to the same idea since it emerged as a field in the late 1990s (Cenoz, Hufeisen, & Jessner, 2001; Williams & Hammarberg, 1998). In academic research in second-language acquisition, the fact that a "second" language learner already knows another language was often not taken into account or completely ignored. It could be that the learner grew up as a bilingual speaker of a minority or heritage language or had acquired another language before. Even if the new target language was thus actually a third or fourth language, it was treated as a second language (L2) and any comparison made with that "L2" or measures of cross-linguistic influence in the "L2" was related only to the first language (L1) of the learner and the other languages remained kind of hidden. What is obvious in the collection of articles in this special issue is that the authors take all languages in the repertoire of their subjects into account.