Epoxy resins are classically toughened by rubber additives, but the effectiveness of rubber toughening tends to diminish with increasing strain rate, decreasing temperature, and decreasing matrix ductility. In this study, we demonstrate that low loadings of 100-200-nm core-shell rubber (CSR) particulate additives can improve high strain rate (10(4)-10(5) s(-1)) impact resistance by nearly 200 % for epoxy resins with glass transition temperatures T (g) in a range between 60 and 110 A degrees C, without large reductions in T (g) or stiffness. In addition, CSR additives improve low-temperature impact resistance of the epoxies down to 0 A degrees C. Size and surface chemistry of the CSR particles influence the ballistic response, with 200-nm diameter, weakly bound, and poorly dispersed CSR particles providing the greatest toughening performance at low filler loadings and high rates. Impact resistance for a systematic series of CSR-modified epoxies covers a transition from brittle to tough behavior, where the failure mechanism changes with effective fracture resistance. For brittle resins, failure is dominated by initiation of Hertzian cone fracture which depends strongly on fracture toughness K (IC), while for tough resins, failure is dominated by plastic yield at the impact site and is independent of fracture toughness above a minimum K (IC) value of approximately 1.2-1.5 MPa m(1/2). Interestingly, quasistatic mechanical properties are reasonably effective qualitative predictors of high-rate impact resistance, suggesting that the toughening mechanisms of CSR particles are similar over the rates studied here. The insights gained from this study are valuable for design of next generation adhesives, polymers, and polymer composite matrices for lightweight protective applications.