This review presents theoretical models of grain-boundary sliding accommodation mechanisms in deformed nanomaterials. Described mechanisms serve as the special modes of plastic deformation and also can enhance fracture toughness. Accommodation mechanism via stress-driven splitting and migration of grain boundaries is reviewed. The model describing the effect of accommodated and non-accommodated grain-boundary sliding on fracture toughness of nanomaterials is considered. Also, theoretical model of special accommodating mechanism representing crossover from plastic shear into rotational deformation is proposed.