Most societies in today's world are multilingual. 'Language contact' occurs when speakers of different languages interact and their languages influence each other. This book is an introduction to the subject, covering individual and societal multilingualism, the acquisition of two or more languages from birth, second language acquisition in adulthood, language change, linguistic typology, language processing and the structure of the language faculty. It explains the effects of multilingualism on society and language policy, as well as the consequences that long-term bilingualism within communities can have for the structure of languages. Drawing on the author's own first-hand observations of child and adult bilingualism, the book provides a clear analysis of such phenomena as language convergence, grammatical borrowing, and mixed languages.