The addition of minor amounts of fillers that have characteristic dimensions on the nanometer length scale to polymer matrix-based materials has attracted wide attention. A high aspect ratio of nanoelements and extraordinary mechanical properties (strength and flexibility) provide the ultimate reinforcement for the next generation of extremely lightweight but highly elastic and very strong advanced composite materials. However, the resultant physical behavior imparted by such an addition, has been, to date, difficult to quantify, especially for advanced thermoset composites. In an effort to better understand the phenomenological changes across multiple length- and time-scales, we first review currently reported methods of calculating nanoelement reinforced composite mechanical properties. Secondly, we describe recent experimental data along with a multi-scale modeling methodology for the calculation of elastic constants and local/interface properties for systems with statistically homogeneous distribution of embedded nanofillers (nanofibers, nanoparticles, nanoplates, or other heterogeneities, that are either aligned or randomly oriented).