With their potentially extraordinary mechanical, thermal, and electrical properties, Single-Walled Carbon Nanotubes (SWNT)/polymer composites and composite fibers may be the ultimate building blocks for next generation ultra-light-weight, ultra-high performance structural applications. However, the dispersity and processibility of SWNT in polymer matrices have been a challenge because of the strong van der Waal attraction between individual nanotubes and their chemical inertness. Predicated on oleum's (100% H2SO 4 with excess SO3) ability to intercalate between individual SWNT inside SWNT ropes, two types of reinforcing SWNT with much improved solubility and dispersity in common solvents were developed: supra-roped SWNT (SWNT-R) and soluble, ultra-short (length<60 nm), carboxylated SWNT(US-SWNT). SWNT-R hold much improved dispersity in super acid and other solvents, and can facilitate the processing of SWNT/polylmer composites and composites fibers. US-SWNT exhibit up to 2 wt% solubility in common solvents. The availability of SWNT-R and US-SWNT open the opportunities for forming high performance composites, blends, and copolymers without inhibiting their processibility. Studies on the synthesis, processing, properties, and morphology of SWNT-R or US-SWNT/polymer composites and composite fibers have demonstrated the reinforcement efficacy of these SWNT in typical thermoset, thermoplastic and liquid crystalline polymer matrices. The epoxy composite system reinforced with 0.5--1 wt% of US-SWNT has shown an average 15% increase in tensile modulus and 50% increase in tensile toughness over those of the neat epoxy. A linear rule-of-mixture calculation indicates the high reinforcement efficiency of US-SWNT in epoxy matrix. The calculated SWNT's elastic modulus approaches the theoretical value. This processible and high performance US-SWNT/Epoxy resin may serve as a matrix material for advanced fiber composites. A novel solution-processing method was introduced to achieve good dispersion of SWNT-R or US-SWNT in Nylon (6, 6) matrix. In comparison to neat resin, increase in tensile modulus and glass transition temperature were observed with 5 wt% nanotubes incorporation. However, the tensile toughness was significant decreased. An advanced SWNT-R/US-SWNT poly(p-phenylene terephthalamide(PPTA) composite fiber system was developed to realize the ultimate SWNT properties and make them processible by conventional fiber spinning processes.