A key question in developmental research concerns how children learn associations between words and meanings in their early language development. Given a vast array of possible referents, how does the child know what a word refers to? We contend that onomatopoeia (e.g. "knock," "meow"), where a word's sound evokes the sound properties associated with its meaning, are particularly useful in children's early vocabulary development, offering a link between word and sensory experience not present in arbitrary forms. We suggest that, because onomatopoeia evoke imagery of the referent, children can draw from sensory experience to easily link onomatopoeic words to meaning, both when the referent is present as well as when it is absent. We use two sources of data: naturalistic observations of English-speaking caregiver-child interactions from 14 up to 54 months, to establish whether these words are present early in caregivers' speech to children, and experimental data to test whether English-speaking children can learn from onomatopoeia when it is present. Our results demonstrate that onomatopoeia: (1) are most prevalent in early child-directed language and in children's early productions; (2) are learnt more easily by children compared with non-iconic forms; and (3) are used by caregivers in contexts where they can support communication and facilitate word learning.