Abstract: In this paper, we review recent progress in the understanding of a novel dislocation mechanism, named correlated necklace dislocations (CNDs), activated in highly oriented nanotwinned (NT) metals under monotonic and cyclic loading applied parallel to the twin boundaries (TBs). This mechanism was initially revealed to be responsible for the continuous strengthening behavior of NT metals when the TB spacing (λ) is reduced to around 1 nm. It was later found that the presence of a crack-like defect could trigger the operation of CNDs at much larger TB spacings. Most recently, atomistic modeling and experiments demonstrated a history-independent and stable cyclic response of highly oriented NT metals governed by CNDs formed in the NT structure under cyclic loading. CNDs move along the twin planes without directional lattice slip resistance, thus contributing to a symmetric cyclic response of the NT structure regardless of pre-strains imposed on the sample before cyclic loading. We conclude with potential research directions in the investigation of this unique deformation mechanism in highly oriented NT metals.