In building reliability into a system, engineers must address a number of practical needs that will enable them to quantify and compare reliability in engineered systems. (1) One is to be able to compare the reliability of one system to another system. (2) Another practical need is to compare alternate system designs for the purpose of engineering a particular optimal system. The practical, standardized, technical tool for characterizing reliability in systems is system signatures which was created in 1985 and since has developed into a powerful tool for qualifying reliability. It is used in all physical structures and stochastic systems where reliability is an important consideration (e.g., automobiles, bridges, electronic networks, airplanes, etc.) Since the introduction of system signatures in Francisco Samaniego's 1985 paper, the properties of this technical concept have been examined, tested and proven in a wide variety of systems applications. Based on the practical and research success in building reliability into systems with system signatures, this is the first book treatment of the approach. It is, therefore, the purpose of this book to provide guidance on how reliability problems might be structured, modeled and solved. Over the past ten years the broad applicability of system signatures has become apparent and the tool's utility in coherent systems and communications networks firmly established. The book compared actual system lifetimes where the tool has been and has not been used. These comparisons-which have been done over the years-demonstrate the practical, feasible and fruitful use of the tool in building reliable systems. Finally, new results and future directions for system signatures are also explored. .