Since its circulation in two draft versions in 1996, and especially since its commercial publication in English and French in 2001, the Common European Framework of Reference for Languages (CEFR) has come to dominate discussion of L2 curricula, the assessment of L2 proficiency, and L2 teaching and learning in Europe. Although it is widely referred to, however, the CEFR remains relatively little known beyond the summaries of its six proficiency levels presented in the so-called ‘global scale’ and ‘self-assessment grid’. This article summarises the CEFR's content, purpose, and origins; describes its reception, paying particular attention to its impact on L2 teaching and learning (especially via its companion piece, the European Language Portfolio) and on the assessment of L2 proficiency; and concludes with a brief consideration of present challenges and future prospects.