In this chapter, fundamental concepts are presented for probabilistic fatigue life prediction including the influences of nondestructive inspection. A review of the traditional deterministic lifing approach (involving safety factors) is provided and contrasted with the probabilistic approach that involves random input variables and acceptable risk levels. The selection of acceptable risk levels for design and service decisions is discussed in the context of health, safety, and business cost/performance considerations. Application examples are provided for critical rotating equipment commonly found in the aero-engine and energy industries. The concepts presented in this chapter are general and can be applied to life prediction of components and systems in other industries.