We provide a brief review of our recent studiesconcerning the effects of various mechanisms of plasticdeformation of nanocrystalline materials on their fracturetoughness. We consider both conventional deformationmechanisms, such as lattice dislocation slip, and the deformationmechanism pronounced mostly in nanocrystalline solids, such asgrain boundary (GB) sliding and migration. We demonstrate thatwith a decrease in grain size, the effect of conventional latticedislocation slip on fracture toughness enhancement significantlydecreases. At the same time, for nanocrystalline solids withsmallest grain size fracture toughness can be increased due to GBsliding and migration. This implies that a transition from latticedislocation-mediated toughening to GB-deformation-producedtoughening can occur at a critical grain size in nanocrystallinesolids.