John Sinclair's Idiom Principle famously posited that most texts are largely composed of multi-word expressions that "constitute single choices" in the mental lexicon. At the time that assertion was made, little actual psycholinguistic evidence existed in support of that holistic, "single choice," view of formulaic language. In the intervening years, a number of studies have shown that multi-word expressions are indeed processed differently from novel phrases. This processing advantage, however, does not necessarily support the holistic view of formulaic language. The present review aims to bring together studies on the processing of multi-word expressions in a first and second language that have used a range of psycholinguistic techniques, and presents why such research is important. Practical implications and pathways for future research are discussed.