Interjections, the words that come between sentences, are easily overlooked and usually treated as peripheral to the language sciences. This review surveys work from disparate disciplines that suggests an inversion of perspective: from interjections as marginal items to interjections at the heart of language. Around one out of every seven turns in conversation is an interjection, and the most common ones are not the involuntary grunts that typically feature in examples; instead, they form a small set of agile and adaptive interactional tools that streamline everyday language use. Continuers like mmhm help people co-construct complex interactional structures, repair initiators like huh? help people calibrate mutual understanding on the fly, and change-of-state tokens like oh display knowledge as it evolves in interaction. Interjections emerge as words that help us talk and think, scaffolding the complexity of language as we know it. Expected final online publication date for the Annual Review of Linguistics, Volume 10 is January 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.