Fracture in nanocrystalline metals is shown to depend on grain size. If the grain size is large enough, nanocrystalline metals fracture with a similar mechanism to coarse-grained materials -with extensive reduction in area and a ductile mode. Grain sizes below a critical value result in brittle fracture, via the breaking of atomic bonds (see Figure and cover). On the nanoscale, the cracks follow a mainly intergranular path.