The current trend in assessment of economic efficiency of power system is towards the introduction of reliability inputs and outputs into risk management programs that are able to assist the regulator and power utility in undertaking decisions concerning system planning and operation. This book, therefore, provides a new framework of risk assessment of power systems which combines the precision of a probabilistic method with the flexibility and correctness of customer and interruption parameters. The framework is presented in the form of case studies and a new reliability worth index is derived. The risk-based approach applied to the interpretation of risk-based interruption cost (RBIC) index significantly influences the perception on the network's reliability performance and allows the uncertainty in a network planning or operation decision to be quantified. Use of the new reliability worth index offer more flexibility in reliability worth assessment and produce more accurate results. It can be used in all areas of power system reliability worth assessment which have always been exclusive domain of the average process.