This chapter focuses on covalent sidewall functionalization of single-walled carbon nanotubes (SWNTs) and discusses how the methodology generates the ability to solubilize and disperse these nanotubes into polymer matrices. Covalent functionalization of the SWNTs has significantly expanded the utility of the nanotube structure. The National Nanotechnology Initiative has defined nine thrusts called grand challenges aimed at rapidly converting scientific discoveries into innovative technology. Several SWNT sidewall functionalization methodologies exist, and they all have one thing in common: a highly reactive intermediate is necessary to attack the carbon nanotubes. Covalent functionalization of carbon nanotubes gives researchers the ability to manipulate these entities in typical organic solvents or even in solvent-free conditions. Since selective reaction chemistries are available that can react preferentially with SWNTs based on nanotube type, functionalization allows the manipulation and separation of highly enriched SWNT fractions.