Effects of nanoparticle addition on the fracture toughness of particle reinforced polymers were investigated. The matrix material is a typical low cost toughened epoxy, SC-15. The nanoparticles used were ZnO ceramic particles, with nominal diameters of 53nm and 75μm. The nanocomposites were fabricated using a shear-mixing device, and fracture toughness measurements using a four point bend apparatus were conducted on volume fractions ranging from 0-4 vol%. A maximum increase in critical stress intensity factor of 80% above the neat resin samples was achieved for the composites reinforced with 4 volume percent nanoparticle filler. Studies using a Focused Ion Beam (FIB) were conducted to investigate the toughening mechanisms of particle-reinforced polymers. Cantilever beams were created over two different length scales (~1mm and ~10μm) and bent using an Omniprobe device in-situ in the FIB. Using this method both the nanoparticles and the crack can be imaged simultaneously and results can be compared with common assumptions regarding the behavior of cracks propagating within particulate filled composites. Experimental results were compared to three hypotheses in an attempt to explain the increase in toughness that can be observed in a typical nanocomposite system. In addition two computational models are presented as a first step in modeling the behavior of the system.