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Models are in wide-spread use in the software development lifecycle and model-driven development even promotes them from an abstraction of the system to the description the system is generated from. Therefore it is increasingly important to collaborate on models. These models can range from requirements models over UML models to project management models such as schedules. Tool support for collaboration on models is therefore crucial. Traditionally Software Configuration Management (SCM) systems such as RCS [9] or Subversion [10] have supported this task for textual artifacts such as source code on the granularity of files and textual lines. They do not work well for graph-like models with many links since the granularity needed to support them is on the level of model elements and their attributes. For the design of a novel SCM system addressing these requirements it is essential to define how conflicts on models are detected and how they can be resolved. In this paper we present an approach to conflict detection and resolution on models. We employ operationbased change tracking and therefore detect conflicts based on operations. For conflict resolution we propose an integration of SCM with techniques from Rational Management to effectively resolve conflicts. MotivationWith an ever growing number of increasingly large models that are maintained for increased periods of time, SCM systems for models are becoming more and more important. Model-driven development is putting even more emphasis on models since they are not only an abstraction of the system under development but the system is (partly) generated from its models. Change pervades the entire software life cycle. Requirements change when developers improve their understanding of the application domain, the system design changes with new technologies and design goals, the detailed design changes with the identification of new solution objects and the implementation changes as faults are discovered and repaired. These changes can affect every work product including models. It is widely recognized that software configuration management (SCM) is crucial for maintaining consistency among, while minimizing the risk and cost of changes to all of these artifacts [11].While many SCM systems exist for source code and other textual artifacts, their support for models with a graph structure is very limited. The traditional SCM systems are geared towards supporting textual artifacts such as source code. Therefore changes are managed on a line-oriented level. In contrast, many software engineering artifacts are not managed on a line-oriented level and therefore a lineoriented change management is not adequate. For example adding an association between two classes in a UML class diagram is neither line-oriented nor can the change be managed in a line-oriented way. A single structural change in the diagram will be managed as multiple line changes by traditional SCM systems. Nguyen et al. describe this problem as an impedance mismatch between the flat textual data models o...

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