This chapter discusses reinforcement theories in rubber. The main intention is to gain the relationship between disordered filler structures and the reinforcement of elastomers which is discussed mainly for the static and dynamic (shear or tensile) modulus. It is recognized that the classical approaches to (filled) rubber elasticity are not sufficient to describe the physics of such disordered systems. Instead, different theoretical methods have to be employed to deal with the various interactions and, consequently, reinforcing mechanisms on different length scales. This chapter explains hydrodynamic reinforcement and the role of polymer-filler interface, filler networking and reinforcement at small strain, the dynamic flocculation model, stress softening and filler induced hysteresis. Filler networking in elastomer composites can be analyzed by applying TEM-flocculation- and dielectric investigations. This provides information on the fractal nature of filler networks as well as the morphology of filler-filler bonds. Schematic views of the mechanical equivalence between a CCA-filler cluster and a series of soft and hard springs, and decomposition of filler clusters are presented. The quasistatic stress strain data (up-cycles) at different prestrains of silica filled rubbers can be well described in the scope of the dynamic flocculation model of stress softening and filler-induced hysteresis up to large strain. The presence of rigid filler clusters, with bonds in the virgin, unbroken state of the sample, give rise to hydrodynamic reinforcement of the rubber matrix.