This thesis primarily documents the development and application of a novel technique, which involves the usage of a silicon micro-mechanical device that operates in conjunction with a quantitative nanoindenter within an electron microscope, for the mechanical characterization of nanomaterials and interfaces in composites. The technique was used to conducted tensile tests on individual pristine, nitrogen doped and sidewall fluorinated multi-wall carbon nanotubes (MWNTs), which were found to exhibit varied load-bearing abilities and unique fracture modes. The technique was also used to perform single fiber pullout experiments to study the MWNT/polymer (epoxy) interface. Interfacial failure was found to occur in a brittle fashion, in a manner consistent with the predictions of continuum fracture mechanics models. Although an improvement in the interfacial adhesion was observed upon sidewall fluorination of the MWNT reinforcements, the results of the study essentially highlighted the weak nature of the forces that bind MWNTs to an epoxy matrix.