<p>Running is a unique mode of locomotion, a gait with clear ground contact and aerial phases, during which the body experiences dynamic deformations at various scales. At each stride, the centre of mass lowers and rises under the action of articulated body segments with muscles and tendons stretching and shortening, inducing tissue loading. The capacity to resist such deformations, to dissipate or transmit forces or to store and release energy are all commonly lumped together within the concept of stiffness. Stiffness is commonly considered to be a major performance factor in running as it may impact running economy. This review (i) provides a brief overview of the various definitions and methods of measurement of the lower limbs' stiffness in the context of running; (ii) synthesizes the current understanding of how stiffness, measured at the various scales, relates to running performance and economy; (iii) analyses the effects of running-induced fatigue on stiffness and their subsequent consequences on running performance and (iv) explores how stiffness can be adjusted to enhance running performance via acute strategies (equipment, warm-up) or long-term training. Current evidence suggests a direct link between stiffness and running economy, but the underlying mechanisms remain unclear across measurement scales. Despite the variety of methods available, stiffness at the tissue level is still underexplored, particularly in relation to performance and running-induced fatigue. More research is needed to identify an optimal stiffness level and clarify how acute or chronic interventions may beneficially—or detrimentally—modulate stiffness and running performance.</p>