The aim of this article is to give an up-to-date picture of study of the role of the background languages (the first language, L1, and the second language, L2) in third language (L3) acquisition, mainly in the two areas of vocabulary and syntax. These seem to be the two linguistic levels on which there has so far been most research concerning cross-linguistic influence (CLI). Lexical CLI and syntactic transfer have in most cases been studied separately, but as we will see studies indicate that L3 learners seem to rely on both vocabulary and grammar from their background languages, at least to some extent. The role of the background languages in morphology and phonology has been less investigated in L3 studies, although there are a few studies that indicate that L3 performance can reflect activation of previously acquired languages at these linguistic levels too. The paper also includes a survey of neurolinguistic approaches to multilingualism and discusses how these findings can contribute to the understanding of transfer in L3 acquisition.