The purpose of this proposal is to develop a new design approach to quantify the reliability of aerospace structures. In this approach, the “Level of Safety (LOS) of an existing structural component is determined based on a probabilistic assessment of in-service accumulated damage and the ability of non-destructive inspection methods to detect such damage. Specifically, the discrete LOS for a single inspection event is defined as the compliment of the probability that a single flaw size larger than the critical flaw size for residual strength of structure exists, and that the flaw will not be detected. The cumulative LOS for the entire structure is the product of the discrete LOS values for each damage type Present at each location in the structure. This approach can be utilized to develop a design process which evaluated the equivalent LOS of an existing structure, and use this value in the design of a new structure which matches or exceeds the existing LOS value. The LOS method enables the characterization of uncertainty associated with damage accumulation, inspection reliability and residual strength of the structure.