This course offers an in-depth introduction to the theories and models of first, second, third, and multilingual language acquisition. We will explore topics related to language contact, such as progressive and reverse cross-linguistic influence, the effects of exposure and age, language attrition, metalinguistic awareness, and phenomena related to language distance. For instance, we will consider whether multilinguals who speak closely related varieties (e.g., dialects) differ from those who speak more distantly related varieties (e.g., languages from different language families). Additionally, the course will examine multilingual contexts involving both globally dominant and globally less widespread languages, with a particular focus on how L2 English influences the vocabulary and grammar of domestically dominant but globally smaller L1s. We will analyze a range of multilingual scenarios, including heritage language acquisition, second and third language acquisition, and language combinations involving English, Icelandic, German, Polish, and Norwegian. The course will also delve into neurolinguistics, exploring the cognitive and neurological effects of multilingualism on the brain.