This course will explore the consequences of the use of two or more languages in an individual or a community. The first half of the course covers individual bilingualism, including experimental research in neurocognition, cognitive advantages or disadvantages of bilingualism, debates over a critical age for language learning, effects of early and late bilingualism, and language attrition. The second half of the course covers aspects of societal bilingualism, including language planning, attitudes, language change through contact, code-switching, and bilingual identity. Throughout the course, the interaction of cognitive and social forces will be emphasised, as will links to linguistic and sociolinguistic theory.