In this document, we introduce a system model as a semantic domain for the Unified Modeling Language (UML) [OMG07a, OMG07b]. The system model is supposed to form a possible core and foundation of the UML semantics definition. For that purpose, the definitions in this document are targeted towards UML which means that central concepts of UML have been formalized as theories of the system model. This document is structured as follows: In the rest of Chapter 1, we discuss the general approach and highlight the main decisions. This chapter is important to understand the rest of this document. Chapter 2 contains the definition of the structural part of the system model. Chapters 3 and 4 contain the control and communication related definition definitions which form the basis to describe the state of a system in Chapter 5. Two variants of state transitions systems are introduced to define object behavior in Chapters 6 (event-based) and 7 (timed). Chapter 8 concludes the document. This document is the second version of the system model which is the result of a major 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. The first version of the system model can be found in [BCR06, BCR07a, BCR07b].