In recent times,system reliability modeling and evaluation of system reliable measures and optimum design of redundant system have become paramount interest in the field of reliability. This book deals with new and improved optimum spares redundancy models for various redundant systems such as N-modular redundant system (NMR), Triple modular redundant system (TMR) and General k-out-of-n: G redundant systems. The modified optimum designing of redundant system in the presence of 'individual' as well as 'lethal common cause shock failures' with specific emphasis on failure rate and time of operation of the system is included in this book. In this book author discusses the new optimum sparing redundancy model and suggests the optimum redundant spare (s*) as a function of failure rate, cost of components and time of operation of the system unlike the earlier models. The model gives optimal reliability with optimal (minimum) number of redundant spares units with minimum cost. Redundancy is one of the techniques to improve the reliability of the system. The book is useful, to those who are interested in cost effective optimal redundancy for improvement of reliability of the system.