This is a Ph.D. dissertation. 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. Today simulation systems play an increasingly important role in our society, which are rapidly becoming the primary tool for crucial decision-making processes during engineering design, test and evaluation of new systems, even safety-critical systems, and in training of people operating these systems. With this increasing reliance on simulation results it is more than ever important to know how well a simulation corresponds to reality in order to ascertain that the risks involved in using the simulation results are within acceptable limits. Despite these observations and the enormous advancements in simulation hardware and software, the ability to characterize, qualify and quantify the level of simulation fidelity is still a largely uncultivated area. An area in which there exist many incomplete, inconsistent and widely scattered views, concepts and approaches to fidelity. What is primarily lacking is the absence of a systematic and general applicable simulation fidelity assessment methodology, which is based on a sound unifying theory for fidelity and associated practices. 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.