We present recent results of theoretical investigations concerning the hydrodynamic reinforcement contribution in different elastic composite systems: elastic matrix plus W rigid filler particles with fractal structure (e.g., carbon black aggregates); (ii) spherical core-shell particles with soft core and hard shell; (iii) spherical core-shell particles with hard core and soft shell (e.g., carbon black particles with bound rubber). Detailed predictions are made on the resulting modulus of the composite systems.