In the literature of generative grammar, idiomaticity has been widely identified with noncompositionality. Such a definition fails to recognize several important dimensions of idiomaticity, including, among others, conventionality and figuration. We propose to distinguish IDIOMATICALLY COMBINING EXPRESSIONS (e.g. take advantage, pull strings), whose meanings—while conventional—are distributed among their parts, from IDIOMATIC PHRASES (e.g. kick the bucket, saw logs), which do not distribute their meanings to their components. Most syntactic arguments based on idioms are flawed, we argue, because they treat all idioms as noncompositional. A careful examination of the semantic properties of idioms and the metaphors that many of them employ helps to explain certain intriguing asymmetries in the grammatical and thematic roles of idiomatic noun phrases.