The mechanical failure of a material, whether tensile failure, tear, abrasion or fatigue, always occurs by a process of rupture propagation, and this paper discusses the strength properties of reinforced rubbers in terms of this process. The effect of fillers and other reinforcing systems on rupture is illustrated and the microscopic role of filler particles discussed. The high rupture resistance of reinforced rubber is attributed to a reduction of stress at the tip of the propagating crack. Model experiments on both unfilled natural rubber and carbon loaded SBR show how such a reduction in stress can result from mechanical hysteresis in the elastomer. This new understanding of the influence of hysteresis on rupture propagation provides a firm basis for the explanation of strength-type reinforcement by fillers.