This book offers an introduction to the many facets of multilingualism in a changing world. It begins with an overview of the multiplicity of human languages and their geographic distribution, before moving on to the key question of what multilingualism actually is and what is understood by terms such as 'mother tongue', 'native speaker', and 'speech community'. In the chapters that follow, Florian Coulmas systematically explores multilingualism with respect to the individual, institutions, cities, nations, and cyberspace. In each of these domains, the dynamics of language choice are undergoing changes as a result of economic, political, and cultural forces. Against this background, two chapters discuss the effects of linguistic diversity on the integration and separation of language and society, before a final chapter describes and assesses research methods for investigating multilingualism. Each chapter concludes with problems and questions for discussion, which place the topic in a real-world context. The book explores where, when, and why multilingualism came to be regarded as a problem, and why it presents a serious challenge for linguistic theory today. It provides the basic tools to analyse different kinds of multilingualism at both the individual and society level, and will be of interest to students of linguistics, sociology, education, and communication studies.