, where a product structure view is needed. For example, when the sales department has identified the concrete system to be offered to the customer, only the product structure is needed for calcu- lating prices and for extracting lists of components to be manufactured, to be tested and to be sent to the site for installation. Although the product structure approach to information management (of system models) has its benefits and justifications when it comes to the provisioning of con- crete systems, this approach was always inefficient from a systems engineering point of view. By introducing sysnet modelling as the primary "tool" for systems engineering, a model management approach is needed. Requirements on a Modelling Language for Systems Mod- elling Without doubt, the success of any modelling method depends to a large extent on the user-friendliness and quality of the modelling tool. No matter how suitable the modelling language is, if potential users must only use the modelling language as a conceptual tool and draw their models by hand or by general-purpose drawing tools (like Microsoft PowerPoint or Adobe FrameMaker), only a few enthusiasts will actually use it. Therefore the potential for improvements in productivity, qual- ity and lead-time in developing complex systems will not be gained. This book is not about modelling tools, but about modelling principles and a modelling language for systems A modelling language can, however, be more or less suited for designing good modelling tools. For example, a language that is built on fuzzy and ambiguous concepts does not lend itself easily to tool design. Different tools handle the concepts and constructs of the language in different ways, thus making human interpretation of the models ambiguous and models that are constructed in different tools incompatible. Such a language will no doubt also be more complex, and therefore more difficult to understand and less appealing to users, than is optimal. Therefore we shall discuss the requirements for a modelling lan