Resin composites are widely used in esthetic restorative dentistry. Since their introduction in the mid-1960s, these composites made steady gains in popularity. Their increased use is attributed to their excellent biocompatibility, absence of taste, odour, tissue irritation and toxicity, insolubility in body fluids, easy operation, excellent aesthetic properties, stable colures, optical properties, easy pigmentation, low cost and repairability. The composite resins in current use still suffer from several shortcomings such as poor mechanical properties. In order to improve these properties, microparticles have been used as fillers for a long time. However, the inadequate mechanical properties of resin composites remain problematic. Recently, researchers have utilised nanoparticles as dental composite fillers. This article reviews the relevant literature on the mechanical properties of polymer dental composites filled with micro- and nano-scale particles. The effects of particle size on fracture toughness, flexural strength, and hardness were examined with emphasis on other important factors for improvement. The second section focused on the toughening mechanisms of particulate-polymer composites.