An overview of theoretical models of plastic deformation and diffusion processes in nanocrystalline (nano-grained) nanocrystalline bulk materials and nanocomposite coatings is presented. The key experimentally detected facts – abnormal Hall-Petch relationship, grain rotations, plastic flow localization, high-strain-rate superplasticity, etc. – in nanocrystalline materials are discussed. Special attention is paid to theoretical models which describe such plastic deformation mechanisms as lattice dislocation slip, grain boundary sliding, grain boundary diffusion creep, triple junction diffusion creep and rotational deformation in nanocrystalline materials with emphasis on competition and interaction between these deformation mechanisms and explanation of the key experimentally detected facts.