Outstanding mechanical performance in polymer nano-carbon composite fibers may be attributed to unique interfacial morphologies existing in the materials, which are formed during processing. Fundamental knowledge as to how these regimes evolve during the processing of the nanocomposite fibers is lacking. In order to continue making progress toward producing mechanically exceptional nano-composites, fundamental and focused studies of interfacial property-structure relationships are necessary. Research as it relates to understanding the development of interfacial zones in the composite fiber as a function of nano-carbon morphology is discussed. In addition, stress transfer analysis of the composite as it relates to these interfacial structures is also discussed. Major characterization tools utilized for this research include wideand small-angle X-ray diffraction, mechanical and thermal analysis, as well as high-resolution microscopy studies. Knowledge of how these interfacial regions form will most critically affect the overall nano-composite morphology leading to high-performance composite fibers that exhibit properties approaching the theoretical limits (i.e. tensile strength 7 to 15 GPa, and modulus 100 to 300 GPa).