Synthetic routes for the preparation of microgels with defined particle size, crosslinking density and surface chemistry are described. The mechanical effects caused by incorporating such systems in crosslinked rubber (NR and SBR) are described. The mechanical effects are qualitatively related to the state of clustering which is governed by the particle size and the interfacial tension (delta-parameter difference). The reinforcing effects on Young＇s modulus and the dynamic shear modulus are explained against a comprehensive theoretical background which assumes kinetically controlled cluster-cluster aggregation (CCA) of colloidal filler particles dispersed in entangled polymers. This leads to a control of rubber material properties by chemical means.