This book provides a comprehensive, research-based account of how people learn a second/foreign language and shows how classroom practice can be organised around research-based principles. In the first part, the book provides up-to-date insights into the cognitive, motivational, and emotional dimensions of learning an additional language. In the second part, ten principles of high-quality additional language teaching are introduced and illustrated by a wealth of authentic, classroom-based examples. The book also explores implications for curriculum design and the assessment of additional language competences. A separate chapter is devoted to the ways in which innovation in language education can be fostered. Throughout the book, the question is addressed whether additional language teaching should primarily focus on meaningful tasks, form-based practice, or the integration of both. This book is a must-read for all those who are interested in improving the quality of second and foreign language education.