Surface-modified silica nanoparticles, 20 nm in size and with a very narrow particle size distribution, have been available as concentrates in epoxy resins in industrial quantities for the last 10 years. They can be used in epoxy resin formulations to improve many different properties, including the strength, modulus, toughness, and fatigue performance. In this review, I examine the literature published in the last decade, compare the results with a focus on the mechanical properties, and discuss the mechanisms responsible for property improvements. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1421-1428, 2013