We provide a brief overview of theoretical models that address twinning and detwinning processes in nanocrystalline and ultrafine-grained metals. In particular, we discuss and compare special mechanisms for formation of nanoscale deformation twins at grain boundaries (GBs) in nanocrystalline and ultrafine-grained materials. The twinning mechanisms represent (i) the successive events of partial dislocation emission from GBs; (ii) the cooperative emission of partial dislocations from GBs; and (iii) the multiplane nanoscale shear generated at GBs. In addition, we examine the detwinning mechanism in ultrafinegrained nanotwinned metals through stress-driven migration of incoherent boundaries of nanoscale twins. We demonstrate that detwinning of ultrathin twins can occur at very low stresses, while detwinning of thicker twins requires high applied stresses, and these theoretically revealed conclusions are well consistent with recent experimental data.