Every teacher knows that learners are notoriously variable in how successful they are at acquiring a new language. This interdisciplinary volume questions what it is that makes each of us good or bad at learning a second language. Offers a broad overview of current theories, key findings, and methodological approaches in the field. Brings together research from language teaching and assessment, psycholinguistics, and the neurobiology of language. Provides a sound empirical basis for the development of assessment tools and teaching strategies, and sheds new light on the language learning process. Investigates how people differ from each other in how they approach language learning, and in doing so goes beyond other studies which focus primarily on the behavior of groups of learners.