Surface-modified silica nanoparticles, 20 nm in diameter and with a very narrow particle size distribution, are available as concentrates in epoxy resins in industrial quantities since 10 years. They improve many different properties like strength, modulus, toughness and fatigue performance. Some of these improvements can be found for fiber-reinforced composites as well. In this review, the research results obtained with commercial silica nanoparticles published in the last decade are studied, results are compared with a focus on mechanical properties and the mechanisms responsible for the property improvements are discussed. Silica nanoparticles present only in the interface between fibers and resin matrix might be a very promising approach for future cost-sensitive applications.