Carbon nanotubes (CNTs) have excellent mechanical strength, thermal conductivity, and electrical conductivity. These properties make them particularly interesting for high performance fiber applications such as lightweight cables and wires, soft biological implants, next generation ballistic protection, and wearable electronics. Initial efforts to develop strong and conductive CNT fibers were slow due to limited CNT production and lack of a suitable solvent due to the strong van der Waals forces between CNTs. However, significant progress in CNT fiber production came from the development of gram-quantity synthesis from the high pressure carbon monoxide (HiPCO) growth process and demonstration of CNT fiber spinning with superacid solvents. Since these developments in the early 2000’s, tensile strength and electrical conductivity of CNT fibers have increased on average ∼20% per year. Research conducted in this thesis has continued this trend and produced CNT fibers with a tensile strength of 4.2 GPa and an electrical conductivity of10.9 MS/m. These properties are now competitive with high strength fibers such as carbon fiber and aramid fibers as well as metal conductors where weight savings, flexibility, or thermal conductivity are important parameters are important parameters. To improve CNT fiber performance, this thesis studied the purification of CNTs for improved solubility in chlorosulfonic (CSA), and the effect of CNT characteristics on fiber performance. This work demonstrates that CNTs with fewer impurities produce fiber with higher electrical conductivity. However, more intense purification (furnace oxidation) decreases the aspect ratio (length of the CNT/diameter of the CNT) which decreases both tensile strength and conductivity. Therefore, purification conditions must be carefully considered to optimize fiber properties. Furthermore, it was found that lower CNT concentration in the spin dope increased tensile strength of CNT fibers. This enhancement in strength is believed to be the result of improved CNT bundle structure within the fiber. Additional improvements in strength and electrical conductivity were also achieved by decreasing the angle of the inlet cone of the spinneret. This result suggests that fiber properties could be further improved by increasing the path length of the spinneret to allow for additional stress relaxation of the solution before coagulation. Finally, this thesis demonstrates that CNT fibers can used as wearable, textile electrodes. CNT fibers were plyed into thread and sewn with a standard sewing machine into textiles to form soft electrodes. These electrodes were able to obtain high quality electrocardiograms (EKGs) on par with commercial wet electrodes. Furthermore, we show that CNT fiber can also be used as transmission lines to carry signal from the recording site to standard electronic components. These results demonstrate that CNT fiber is the ideal material for wearable electronics because it is conductive, soft, washable, and easy to integrate into clothing.