Presents substantial evidence for the foundational role of second language linguistic knowledge over and above that of motivation and self-efficacy This book presents a comprehensive study into the components of language proficiency among 302 adolescent learners of French and Spanish in state-funded schools in England. It fills a significant gap in research on language learning in highly instructed settings and offers critical insights into current debates in modern language learning, teaching and assessment, both in England and internationally. It presents data on various constructs associated with proficiency - linguistic knowledge, skills, motivation and skill-specific self-efficacy - and explores how these relate to each other. Alongside their findings, the authors provide online summaries of each chapter and freely available materials for use in research, teaching and testing. This book will be of interest to students, trainee and practising teachers, teacher educators, and researchers of instructed second language learning, teaching and testing. This book is open access under a CC BY NC ND licence.