One of the most important phenomenon in material science is the reinforcement of rubber by rigid entities, such as dispersed particulate filler or phase-separated organic domains. In order to impart significant reinforcement, the size of the hard phase must be small, much less than a micron. The basis of this requirement is a major focus of this short review. Furthermore, the roles of energy dissipation and crack deflection in rubber reinforcement are considered. The final part of the review deals with nano-composite rubbers, in which rigid domain size is in the range of 1–10 nm.