Life extension of high cost components until measurable damage is detected can result in marked reductions in total cycle costs. The life extension strategy for turbine disks is based upon nondestructive inspection to detect defects, usage and stress analysis to define requirements, and fracture mechanics analysis and testing to evaluate the severity of any defects under future usage. Because there are uncertainties and inaccuracies in the inspection, analysis, testing and definition of past and future usage, the selection of the optimum life extension strategy requires quantitative evaluation of the costs and risks associated with each uncertainty during life extension. This paper summarizes recent developments in the basic methodologies necessary to quantify reliability. Specific examples are described which illustrate the concepts and payoff possible as well as the relative importance of inspection, analysis, and usage uncertainties on the optimum life extension strategy.