The potential of using exfoliated graphite nanoplatelets, xGnP TM, as a reinforcement that can produce multifunctional polymer composites was explored. xGnP–polypropylene (PP) composites fabricated by melt mixing using a twin-screw extruder followed by injection molding were investigated for their thermal, viscoelastic and barrier properties as a function of xGnP concentration and aspect ratio. These properties of the xGnP–PP composites were compared to the properties of composites made with PAN-based carbon fibers, VGCF, carbon black and nanoclay. Results indicate that when oriented properly, the xGnP will not only stiffen the composite but also reduce the coefficient of thermal expansion in two directions rather than in one as in the case of aligned fiber composites. Furthermore, the large aspect ratio of xGnP, even at low loadings, increases the oxygen barrier of PP at least as effectively as the commonly used nanoclays and finally, addition of xGnP significantly enhances the thermal conductivity of the polymer matrix.