Eclogite is an unusually dense metamorphic rock, typically of basaltic composition, that represents a recorder and/or driving force for planetary-scale processes and evolution. Eclogite-facies metamorphic conditions encompass a wide range of pressure–temperature space from deep crustal to mantle depths, reflecting tectonic setting. Eclogite in Pacific-type orogenic belts records the densification of subducted oceanic crust that drives plate tectonics, and the attendant fluid release that drives both magmatism in the overlying arc and within-slab seismicity below. Eclogite in collision-type orogenic belts records continental collision, and eclogite-facies pseudotachylite records deep earthquake processes in subduction settings. In magmatic arcs, crystallization of mantle-derived basaltic magma produces eclogite-facies metacumulates at depth; delamination of these highdensity bodies leads to net growth of lower-density, silicic continental crust. In kimberlites, most eclogite xenoliths and eclogitic inclusions in diamond represent fragments of ancient oceanic crust subducted as deeply as the lower mantle (> ~660 km). Rare eclogite fragments in chondritic meteorites may represent the deep interior of a proto-planetary body. New techniques, new findings, and innovative ideas continue to drive discovery in eclogite-facies rocks in diverse tectonic settings.