Fretting occurs in mechanical or structural joints that were not intended to move, relative to each other, but because of vibrational loads or cyclical deformation, experience minute cyclic relative motion. Fretting-wear develops as a form of wear that retains the majority of the wear debris within the contacting interface. Fretting-wear failure may be defined in functional terms of the normal approach accrued. The magnitude of normal approach due to fretting-wear can be measured in-situ as a function of cycles of operation. These data can be transcribed directly into design curves to enable prediction of impending functional failure in mechanical/structural joints subjected to fretting-wear. Supporting parametric data have evolved from these experimental investigations yielding further insight to the phenomenon of fretting-wear in steels such as SAE 1020, SAE 4340 and SAE 52100. Further research in this branch of tribology will prove of value to designers whose systems reside in a dynamic environment.