It is well-known that the orientation of the reinforcing elements in polymer-based composites plays a vital role in controlling mechanical properties. A number of different techniques can be employed to quantify the orientation of the reinforcement in composites. There is so far, however, no generally-accepted way of quantifying the orientation at the nanoscale of plate-like graphene flakes in a nanocomposite material. In this work, polarized Raman spectroscopy has been employed to characterize, first of all, the orientation of transverse sections of monolayer graphene on a copper substrate. Well-defined Raman spectra can be obtained from these transverse sections even though they are only one atom thick, because of the very strong resonance Raman scattering from graphene. It is found that the intensity of scattering of the Raman band is dependent of the axis of laser polarization when the laser beam is parallel to the surface of the graphene plane and it has been demonstrated that a generalized spherical expanded harmonics orientation distribution function (ODF) can be used to quantify the orientation of this graphene monolayer. Based on this approach, polarized Raman spectroscopy was used to quantify the level of orientation of the graphene flakes in variety different graphene-based materials and nanocomposites. It is demonstrated further how it is possible to relate the degree of graphene orientation to stress transfer to the reinforcement in nanocomposites and, in particular, determine the Krenchel orientation factor for these plate-like fillers from the ODF. It is then shown how his approach can be employed to estimate the effective Young's modulus of the reinforcement in graphene-based nanocomposites and nanocomposites based upon other 2D materials.