Being presented with phrases of the kind, 'take the plunge' and 'write a letter', native speakers of English tend to agree that the former is more idiomatic that the latter. What exactly is it about these two phrases that guide speakers' judgements? Adopting a usage-based perspective, this study addresses the question 'which factors do speakers rely upon when assessing the idiomaticity of a construction. Rethinking Idiomaticity is the first study to bring together a quantitative corpus-linguistic approach and quantitative judgement data to explore the nature of idiomaticity as a complex concept that comprises semantic and formal variation parameters. Wulff's fascinating book is suitable for researchers and postgraduates in the fields of lexicography, phraseology, corpus linguistics and those who are employing quantitative approaches. Cognitive linguists interested in the empirical underpinnings of their theoretical assumptions will also find this required reading.