Disclinations were first observed in mesomorphic phases. They were later found relevant to a number of ill-ordered condensed-matter media involving continuous symmetries or frustrated order. Disclinations also appeared in polycrystals at the edges of grain boundaries; but they were of limited interest in solid single crystals, where they could move only by diffusion climb and, owing to their large elastic stresses, mostly appeared in close pairs of opposite signs. The relaxation mechanisms associated with a disclination in its creation, motion, and change of shape involved an interplay with continuous or quantized dislocations and/or continuous disclinations. These were attached to the disclinations or were akin to Nye's dislocation densities, which were particularly well suited for consideration here. The notion of an extended Volterra process was introduced, which takes these relaxation processes into account and covers different situations where this interplay takes place. These concepts were illustrated by a variety of applications in amorphous solids, mesomorphic phases, and frustrated media in their curved habit space. These often involved disclination networks with specific node conditions. The powerful topological theory of line defects considers only defects stable against any change of boundary conditions or relaxation processes compatible with the structure considered. It could be seen as a simplified case of the approach considered here, particularly suited for media of high plasticity or/and complex structures. It could not analyze the dynamical properties of defects nor the elastic constants involved in their static properties; topological stability could not guarantee energetic stability, and sometimes could not distinguish finer details of the structure of defects.

Disclinations, Dislocations and Continuous Defects - a Reappraisal. M.Kleman, J.Friedel: Reviews of Modern Physics, 2008, 80[1], 61