While complex behavior can be generated through simple systems, as in chaotic and nonlinear systems, complex systems are found where a systems study contains multiple physical objects and interactions. Through the use of hierarchy, we are able to simplify and organize the complex system. Every level within the hierarchy may be refined into another level. System abstraction involves simplification through structural system representation as well as through behavioral approximations of executed model structure. There has been little work on creating a unified taxonomy for model abstraction. We present such a taxonomy and define two major sub-fields of model abstraction, while illustrating both sub-fields through detailed examples. The introduction of this taxonomy provides system and simulation researchers with a way in which to view and manage complex systems.