Repairs the various ways in which people edit and reformulate conversational turns are a characteristic feature of natural dialogue. However, relatively little is known about their overall frequency or distribution in conversation. We present a systematic, quantitative study of patterns of repair in two corpora: ordinary dialogues from the British National Corpus (BNC) and task-oriented dialogues from the HCRC Map Task. We use this analysis to evaluate three hypotheses about patterns of repair 1) social preferences 2) processing demands and 3) dialogue co-ordination. The results show that repair is more frequent in task-oriented dialogue, that use of repair is broadly unaffected by familiarity or mode of interaction but substantially affected by task roles. We argue that the complimentary patterns of repair used by conversational partners support the view of repairs as an integrated, cross-turn and cross-person, system for sustaining the mutual-intelligibility of dialogue.