Key results of our recent investigations on the development of polymer/cellulose nanocrystal (CNC) nanocomposites will be presented and discussed. The base polymers were polypropylene (PP) containing a malleated PP (PPMA), PP containing poly(ethylene-co-vinyl alcohol) (PVOH), used as a compatibilizer, as well as polylactide (PLA). A much finer dispersion, at the nanoscale, of the CNCs in the polymer matrix could be achieved using solution-cast methods. The rheological properties of these nanocomposites were shown to exhibit solid-like behavior and an apparent yield stress at very contents of CNCs. For PPs with two different molecular weights and prepared in the molten state using different temperatures, the tensile modulus of composites containing PPMA and 2 wt% CNCs was improved by about 30% and the tensile strength was increased by 10% in comparison with the unfilled matrices. The tensile modulus of the PP/PVOH nanocomposites containing 5 wt% CNCs was increased up to 47 % compared to the neat PP. However, the best mechanical properties were obtained for PLA/CNC nano-composites using a solution preparation method based on N,N-dimethylformamide (DMF). In dynamic mechanical thermal analysis (DMTA), the storage modulus of PLA containing 6 wt% CNCs increased up to 74% in the glassy region and to 490% in the rubbery region. However, the elongation at break decreased for all PP or PLA/CNC nanocomposites. The Halpin-Tsai model was used to predict the Young moduli of nanocomposites and good agreement with the experimental data was observed for both PP and PLA composites.