The term “compositionality” is rather misleading with regard to idioms since no idiom is completely compositional. A ‘compositional idiom’, in one sense, is an idiom whose meaning is somehow related to the meaning obtained by computing the string literally, and the degree of relation can differ across idioms. In contrast, the principle of compositionality2, the meaning of a composite expression is a function of the meaning of its parts, is a standard principle in logic. Hence, figurative language presents a problem for formal linguistics, and in my thesis no attempt is made to offer an overview of scholarly writing on handling the notion of compositionality in idiomatic expressions from a ‘formal’ perspective. Instead, I will be focusing on how the notion of ‘compositionality’ reflects itself within the areas of psycholinguistics, syntax, and cognitive grammar because a focus on only one of these areas of linguistics would fail to yield a general, adequate account of the compositionality of idiomatic expressions.