Book excerpt: The long-standing view that idioms are formally and thus also semantically fixed has been challenged and many studies have shown that idioms are indeed flexible both structurally and semantically to varying degrees. We often come across examples of creatively used idioms, with their original structure disrupted and their semantics affected in the process. This book investigates the phenomenon of innovation and creativity in phraseology and examines the limits to innovation, i.e. it questions whether modified idioms are choreographed by a set of principles or constraints and whether these principles are coherent. Conceptual Integration Theory is used to analyze modified phraseological units in order to provide insights into mechanisms which regulate their creation and cognitive organization.