Predictions of the worth of transmission systems must take into consideration the ‘hard’ measures of transmission benefits—reductions of required generation reserves and improvements in resources utilization through economy interchanges and improved dispatch—as well as the ‘soft’ measures—reductions of environmental impact, reduction of costs of interruptions and curtailments and the mitigation of resource shortfall due to natural and political events. Procedures are not yet available for the full application of these concepts to planning bulk power systems. The limits to application include the inability to form predictions of the absolute indices of risk and extent of bulk power system failures and the appreciable uncertainty in estimates of the costs of bulk-power-caused interruptions and curtailments of service to customers. There are alternative approaches available for evaluating the worth of reliability of bulk power systems. These approaches make use of relative indices of reliability and make use of security-based criteria for defining bulk power system failure.