Due to the scarcity of “easy slip” systems in hexagonal materials with a hexagonal close-packed structure (hcp), deformation twinning plays a crucial role in determining mechanical properties and texture evolution. In this article, we highlight the current understanding of mechanisms and mechanics of the twinning system \$left\{ {10\bar 12} \right\}\left〈 {10\bar 1\bar 1} \right〉\$, which is commonly activated in all hexagonal materials for nucleation and propagation of deformation twins, twin–twin interactions, solute segregation at twin boundaries, and the development of constitutive models that account for the fundamental mechanisms of twinning/detwinning. Future directions such as characterizing the three-dimensional shapes of twins, the influence of solute atoms on twin propagation, and the influence of twin–twin junctions on mechanical properties of the hexagonal materials are discussed.