In this chapter, we describe some of the key modeling tools that can be used for system modeling. System modeling is the underlying science representing the structure and behavior of an operational entity. When the methodology is applied to commercial entities, the system is an enterprise that can be modeled with a number of sub-models that have user-oriented meta-knowledge structures for communication with human sources of information. The success of the methodology in different areas depends on its representational and explanatory capabilities. These sub-models are used to describe the same enterprise from different viewpoints. Due to the complexity of the models involved (both in terms of the number of components and of their relationships), a modular and incremental modeling approach is often required.