Conventional techniques e.g. Fault Tree Analysis (FTA) and Failure Modes & effect Analysis (FMEA) are in use for working out the realibility of the system. However, these techniques have limitations for complex systems and requires number of assumptions which are based on individual experience and time consuming. Morever, these techniques can not be used for Phased Mission System (PMS). Binary Decision Diagram (BDD) and Ternary Decision Diagram (TDD) are advance techniques which are suitable for any system including Phased Mission System. This book focuses on the development methodology for algorithm and computer programme based on BDD & TDD techniques for working out the realibility of any complex system. The book provides a detailed stock on methodology of working out the realibility of Hydraulic System as a case study using FTA, FMEA, BDD & TDD techniques. The results are compared & explained. It also explains on the new methodology for working out the realibility of system after considering the "Derating" of component with example.