This volume introduces a new concept, `criterial features`, for the learning, teaching and testing of English as a second language. The work is based on research conducted within the English Profile Programme at Cambridge University, using the Cambridge Learner Corpus. The authors address the extent to which learners know the grammar, lexicon and usage conventions of English at each level of the Common European Framework of Reference (CEFR). These levels are currently illustrated in functional terms with `Can Do` statements. Greater specificity and precision can be achieved by using the tagged and parsed corpus, which enables researchers to identify criterial features of the CEFR levels, properties that are characteristic and indicative of L2 proficiency at each level. In practical terms, once criterial features have been identified, the grammatical and lexical properties of English can be presented to learners more efficiently and in ways that are appropriate to their levels.