Covalent functionalization of single-walled carbon nanotubes (SWNTs) has significantly expanded the utility of the nanotube structure. Covalent sidewall functionalization has been employed to increase the solubility of these materials, which allows for the manipulation and processing of these otherwise insoluble nanotubes. Increased solubility leads to better dispersion in polymeric systems. Functionalization can be performed selectively wherein the metallic SWNTs react faster than the semiconductors. This has allowed a separation of carbon nanotubes by type. Covalent sidewall functionalization also allows nanotube-based composite formation where the functional group is well mixed with the polymer matrix. This has led to dramatic increases in the modulus of elastomers while retaining their elongation-at-break properties.