Almost every kind of software development periodically needs to merge models. Perhaps they come from different stakeholders during the requirements analysis phase, or perhaps they are modifications of the same model done independently by several groups of people. Sometimes these models are consistent and can be merged. Sometimes they are not, and negotiation between the stakeholders is needed in order to resolve inconsistencies. While various methods support merging, we need formal approaches that help stakeholders negotiate. We present a formal framework for merging and conflict resolution. It facilitates automatic merging of consistent models, enables users to visualize and explore potential disagreements and identify their priorities, and suggests ways to resolve the priority items.