Systems containing optical fiber have design lives on the order of decades so that models for assessing the mechanical reliability of the fiber must rely on extrapolations from accelerated short term testing. Such extrapolations are only valid if all relevant mechanisms are fully understood. The physical processes giving rise to mechanical degradation are reviewed and it is shown that no single model describes all situations. In particular, strong “pristine” fiber can behave quite differently from weaker fiber. Additionally, two degradation regimes are identified, one which is stress assisted (fatigue) and one which can occur even in the absence of applied stress (aging). Recent advances in understanding these phenomena are discussed and promising areas for future work are proposed.