Models and theories to explain the Hall–Petch relationship are reviewed briefly. Then, a dislocation model to incorporate some characteristic mechanical properties of ultrafine-grained and nanocrystalline metals will be introduced and used to explain some experimental results. The model is based on the idea that dislocations emitted from grain boundaries and bow out into grain interiors during their propagation are responsible for plastic deformation and thermally-activated depinning process at grain boundaries is regarded to be rate controlling. Some implications of the model are discussed in the light of recent experimental results.