In foreign language classrooms we often find that, in addition to their mother tongue (L1), learners already speak--or are learning--at least one other language. As a result, they already have an array of linguistic and cognitive skills that may prove very useful if they are adequately exploited during the language learning process. However, in contrast with the growing interest displayed by researchers in the processes involved in the acquisition of a third or subsequent language (e.g. Cenoz 2001; De Angelis 2007; Rothman 2011), the particular characteristics of multilingual learners often go unnoticed by foreign language teachers (Pauwels 2014). In light of this, we present a survey of key concepts in the field of third language acquisition (TLA) and make a number of suggestions for classroom practice based on current research in the area. We first analyze concepts such as psychotypology (Kellerman 1978, 1983) and cross-linguistic influence. Then, drawing on work done by authors such as Thomas (1988), Jessner (1999, 2006, 2008a) and Carvalho and Silva (2006), we propose that teachers should take advantage of multilingual learners' metalinguistic awareness, for example by adopting strategies such as reactivating prior linguistic knowledge and exploring the formal differences and similarities between the languages present in the classroom.