The issue of high-level language proficiency in other than monolingual contexts can be approached from a variety of perspectives, including linguistic/structural; psycholinguistic/cognitive and sociolinguistic/societal. Bringing together a team of experts, this volume takes a novel empirical approach to the subject combined with an up-to-date understanding of these research areas, to answer two important research questions in the field of second language acquisition: what conditions allow learners to attain an outstanding level of proficiency in a second language, and what factors still prevent them from becoming entirely like first language speakers. Looking at a range of European languages including English, French, Italian, Spanish and Swedish, it provides important insights into second language use at the highest levels as well as in high-proficient mixed language use in multicultural settings. A useful tool for both language teaching and language teacher training, it provides a solid grounding for further study in this important area of research.