Constructing comprehensive operational models of intended system behaviour is a complex and costly task. Consequently, practitioners have adopted techniques that support incremental elaboration of partial behaviour descriptions. A noteworthy example is the wide adoption of scenario-based notations such as message sequence charts. Scenario-based specifications are partial descriptions that can be incrementally elaborated to cover the system behaviour that is of interest. However, how should partial behavioural models described by different stakeholders with different viewpoints covering different aspects of behaviour be composed? How should partial models of component instances of the same type be put together? In this paper, we propose model merging as a general solution to these questions. We formally define model merging based on observational refinement and show that merging consistent models is a process that should result in a minimal common refinement. Because minimal common ref...