Carbon nanotubes (CNT) have attracted a huge interest in both academia and industry because their exceptional mechanical, electrical and thermal properties. In addition, their large aspect ratio, low density and high flexibility make CNT a unique nano-filler to develop advanced rubber materials. Although CNT-rubber nano-composites show enhanced characteristics in comparison to their micro-composite counterparts (e.g. based on carbon black or silica-silane systems), they have not been able to reach the expected properties. In this work, a systematic characterization of CNT-rubber structure has been performed by using a combination of experimental techniques (including advanced 1H time-domain NMR methods) and the last developed analysis procedures in order to improve the understanding about the mechanical and viscoelastic properties of these nano-composites. Combination of uniaxial tensile properties, equilibrium swelling experiments and double-quantum (DQ) NMR experiments has revealed a strong contrast in the constrain density at the CNT interface where the molecular weight between interactions (including cross-links and entanglements) is shorter than in the bulk. Despite the enhanced filler-rubber interaction (in comparison with traditional rubber composites), the main reinforcing factor that govern the mechanical properties of these nano-composites is the filler network. Due to van der Waals interactions in combination with their high aspect ratio, CNT tend to agglomerate inside the rubber matrix into bundles that critically reduce the theoretical shape factor of these nano-composites and consequently, the expected improvement on their reinforcing effect. In addition, the complex hierarchical structure of CNT inside the rubber matrix could partially explain the unexpected high energy dissipation phenomena (observed in the dependence of loss modulus, G¿¿) with the strain amplitude. In the low and intermediate strain regimen (inside the linear behavior), the loss of energy should be related to the shear at the filler-rubber-filler interface and the rupture of the filler network, respectively. However, the high G¿¿ values in the high strain regime should be related to structural factors associated to the rubber matrix but not to filler effects, i.e. reduction in the cross-link density and increasing in the non-elastic network defects (dangling chains, chain ends, loops¿) with the concentration of nano-particles. According the new insights in the CNT-rubber structure, these promising materials could have some limitations to be applied in high-performance tire tread compounds (mainly related to the rolling resistance and fuel consumption) because of the difficulty of dispersion of CNT in rubber matrices (high filler networking), the strong influence of CNT in the vulcanization process (low cross-link density and high network defects) and the nature of filler-rubber interactions (high energy dissipation associated to the rupture of filler-rubber interactions at high strain amplitudes). To overcome these issues, the CNT have been surface-modified with oxygen-bearing groups and sulfur, in order to achieve i) better dispersion of CNT in the elastomeric matrices, ii) formation of covalent bonds between rubber chains and the CNT surface and iii) reduction of network defects.