While the literature on second language acquisition and use is overwhelmingly rich with respect to initial and intermediate stages of development, present knowledge of levels of ultimate attainment that are equal or close to that if native speakers has so far not been presented in a coherent manner. This is what the present volume aims to achieve. In addition to chapters that summarize what is currently known about the grammatical, lexical, and discourse features that continue to exhibit instability at the most advanced levels of second language development, the volume presents overviews of the incipient research on two unique learner populations, polyglots and employees in international call centres. Polyglots, defined as language users who are proficient in six or more second languages, may be considered second language learners par excellence. Call centre employees in economically less developed parts of the world are intriguing in how they cope with the high language proficiency requirements of their job. In conclusion, this book is relevant for all readers - both professionals and students - interested in the development of second language theory. For language teachers, the book provides insights that are profitable in classrooms for advanced learners.