In this document we introduce a system model as the basis for a semantic model for UML 2.0. The system model is supposed to form the core and foundation of the UML semantics definition. For that purpose the basic system is targeted towards UML. This document is structured as follows: In the rest of Section 1 we will discuss the general approach and highlight the main decisions. This section is important to understand the rest of this document. Section 2 contains the actual definition of the structural part of the system model. It is built in layers as described in Section 1. For brevity of the approach, we defer deeper discussions into the Appendix in Section 4. This document is part of a project on the formalization of the UML 2.0 in cooperation between the Queens University Kingston and the Technische Universitäten Braunschweig and München. This version 1.0 is the result of a longer effort to define the structure, behavior and interaction of object-oriented, possibly distributed systems abstract enough to be of general value, but also in sufficient detail for a semantic foundation of the UML. We also wish to thank external reviewers, and especially Gregor von Bochmann, Gregor Engels and Sébastien Gérard for their help.