In this talk, I examine how repairs in adult-child conversations guide children’s acquisition of their first language. Children make self-repairs (repairs to their own utterances) from as early as age one. They repair their own pronunciations, such as fa to fan, based on their auditory representations in memory of the target words they are attempting (fan). For their part, adults systematically check on whether they have understood the children they are talking with. They do so with requests for clarification that are general (mh?, what?) or specific (you hid what?), and with reformulations of what the child appeared to mean (you want water?). Children typically respond to requests for clarification with self-repairs in the next turn. They also use the reformulations as a source of feedback. The contrast between their utterance and the adult’s reformulation helps them identify the error being targeted (negative feedback), and they use the reformulation itself as a model for the conventional version of their less-than-ideal utterance (positive feedback). I describe the use of reformulations in conversations with children acquiring English and French in repairing their errors in phonology, morphology, lexicon, and syntax. I also present two case studies of how reformulations inform children, one of homophonous French verb forms and the other of early Hebrew verbs. I argue that the process of repair plays an essential role in the acquisition of a first language.