Although it has long been observed that some children incorporate unglossable syllables into their early utterances, it has been difficult to integrate these ‘fillers’ into theories of language acquisition. Because they straddle boundaries between phonology and morphosyntax, and between pragmatics and lexicon, they do not fit neatly into linguists' notions about ‘modules’ of language. Fillers have been reported in quite an array of languages, and yet they seem to be more common among learners of some languages than others. Even when language is held constant, children seem to vary immensely as to whether they produce fillers at all. With more researchers reporting fillers in more languages, it seems time to (1) review what we now know about fillers; (2) propose a reasonably unified set of criteria for identifying them; and (3) suggest an approach that will promote their further study.