This chapter summarizes the knowledge of filler modification with diazonium salts, with a view to designing high-performance reinforced polymer composites. Arylation of fillers serves for attaching prepolymers or for triggering polymerization processes. The level of modification is molecular or macromolecular in nature. Both arylated and macromole-modified fillers served to design reinforced polymer composites with unique mechanical and dielectric properties. One important feature is the rheology of formulations prior to cure. Interfacial chemical compositions could be investigated with surface-specific techniques such as XPS and ToF–SIMS. Handpicked cases concern the design of reinforced polymers with silica, zeolite, clays, and carbon allotropes. We demonstrate that fine control over the surface chemical composition of fillers has profound effect on mechanical, thermal, viscoelastic, and electrical properties of end polymer composites.