This book brings together papers dealing with essential issues in applied linguistics and multilingualism that have been contributed by leading figures in these two fields and present state-of-the-art developments in theory and research. The first part includes articles touching on various aspects of multiple-language acquisition, with a particular emphasis on the role of affordances, the interfaces between language and thought, and factors influencing the process of language learning. Part Two deals with individual variations in the acquisition of additional languages, focusing in particular on the impact of such variables as age, aptitude, motivation and learning deficits. Finally, Part Three presents contributions illuminating key issues in the acquisition of different subsystems and skills, such as grammar, phonology, lexis and writing systems. Thanks to the diversity of perspectives on applied linguistics and multilingualism, as well as the cutting-edge nature of some of the proposals, this edited collection will be an important reference work and a source of inspiration for theorists and researchers.