The creation of the Common European Framework of Reference for Languages (CEFR) has given rise to interest and debate among policy makers, testers, teachers and researchers alike in the reliability and feasibility of the assessment of second language (L2) proficiency. This volume brings together concrete ideas on identifying and measuring L2 proficiency from different branches of SLA research (psycholinguistic, sociolinguistic, corpus-based, applied linguistics) to contribute to a deeper understanding of what it means to be proficient in an L2. The chapters introduce a wide range of tools that are innovative, reliable, and easy-to-use for the evaluation of learners' language level with respect to both productive and receptive skills and provide a variety of answers to the question of how to assess L2 proficiency in a valid, reliable and practical manner. The collection will therefore inspire language teachers, teacher trainers and language testing specialists and help them adapt their assessment practices when necessary, and will also be a valuable resource for postgraduate students and researchers.