Mechanisms responsible for the formation and/or uplift of HP metamorphic terrains form one of the major questions concerning the origin of orogenic rootzones. Eclogites form a common rock-type in HP metamorphic terrains, in which the sodium-bearing clinopyroxene omphacite is a major component. The thermal and deformation-induced omphacite microstructures are reviewed for the following reasons: Thermal induced omphacite microstructures (ie. ordering and exsolution) can be used to determine relative uplift rates of HP metamorphic terrains. Some thermal induced omphacite microstructures also have thermometric properties. In addition omphacite exsolution processes have been subdivided into continuous (intracrystalline) and discontinuous (intercrystalline) precipitation processes. Deformation-induced omphacite microstructures (intracrystalline defect structures and shape fabrics) can be used to determine the rheological properties of eclogites during HP metamorphism. Results indicate that crystal plastic processes (multiple dislocation glide and/or climb) operating in omphacite are the dominant deformation processes and control the formation of omphacite crystallographic preferred orientations (CPO). In addition variations in omphacite CPO reflect strain symmetry differences. Consequently omphacite CPO patterns can put constraints on the tectonic setting for the formation of HP metamorphic terrains.