Cambridge ESOL, the exam board within Cambridge Assessment which provides English language proficiency tests to 3.5 million candidates a year worldwide, uses the Common European Framework of Reference for Languages (CEFR) as an essential element of how we define and interpret exam levels. Many in the UK who are familiar with UK language qualifications may still be unfamiliar with the CEFR, because most of these qualifications pay little attention to proficiency - how well a GCSE grade C candidate can actually communicate in French, for example, or whether this is comparable with the same grade in German. The issues of comparability which the CEFR addresses are thus effectively different in kind from those that occupy schools exams in the UK, even if the comparisons made - over time, or across subjects - sound on the face of it similar. This article offers a brief introduction to the CEFR for those unfamiliar with it.