Abstract
<jats:title>Abstract</jats:title> <jats:p>Multilingualism involves a dynamic interaction between grammars that begins at the very first contact with a new language and remains crucially important throughout its acquisition. To understand how the influence of previously acquired languages shapes the course of third or further language acquisition, researchers working from a cognitive perspective have developed models to capture the systematicity of linguistic transfer in multilingualism. This entry surveys the past and current theoretical landscapes, main empirical findings, and latest methodological developments that have increased our understanding of how, when, and to what extent previously acquired languages become involved in the process of incorporating a new grammar into the multilingual brain.</jats:p>