In model based design model fragments are used in everyday work. Concurrent operations on separate parts of a model and communication between stakeholders are some examples. However, the use of model fragments is not always concious, partly because it is typically not covered by the design process. The concept of model fragment is practically undefined, which leads to adopting loose, ad hoc definitions. Thus, communication using model fragments is prone to misunderstandings and creating and using them is fuzzy. This prevents developing of systematic methods and tools. In this paper we clarify the concept and the problem area of model fragments, and discuss how to create and use them. We also present a proof-of-concept implementation for a model fragmentation mechanism and demonstrate it in a small example.