Simulation fidelity is an intrinsic element of any simulation system, one that all its developers and users have to deal with one way or the other. It is commonly recognized by the modeling and simulation community that simulation fidelity is an essential vehicle in properly assessing the validity and credibility of simulation results. Furthermore, fidelity is one of the main cost-drives of any model or simulation development. Rigorous assessment of fidelity is, however, one of the most difficult and hard to grasp issues within the model and simulation community. Substantial and exhaustive research endeavors in this area are very limited. Due to this, simulation fidelity still remains a hardly touched upon and rather uncultivated area. This thesis tries to fill this void by the analysis, extension and integration of existing simulation fidelity approaches into a single unified fidelity theory and practice. All this is done from a general simulation system life cycle perspective, not limited by any specific application or problem domain aspects. The foundation for this developed unified fidelity framework comprises a precise mathematical formulation for fidelity and the fundamental concepts underlying its characterization and measurement. The unified fidelity framework is completed with a fidelity management process model outlining a series of generic stages, activities and tasks, which together provide a structured but generic approach to properly integrate and apply all other unified fidelity framework elements in the simulation system development and validation process.Aerospace Engineerin