People are very adept at recognizing when something they said has been misunderstood by a conversational partner and at recognizing when they themselves have misunderstood something that was said earlier in the conversation. In either case, they will usually say something to repair the situation and regain mutual understanding. If computers are ever to converse with humans in natural language, they must be as adept as people are in their ability to detect and repair both their own occasional misunderstandings and also those of their conversational partner. The processes through which conversational repairs take place include negotiation, collaboration, and construction of meaning. By modeling the mechanisms for collaboration and negotiation that natural language uses that we will be able to develop mechanisms for semantic interoperability in complex non-linguistic forms of communication.