Physical juxtaposition of chemically disparate rocks, and the mobility of “fluids” at up to tens of kilometer scales, lead to myriad metasomatic effects in subduction zones, with larger scale manifestations including the metasomatism related to mass flux leading to arc magmatism and convergent margin volatiles cycling. Subduction-zone metasomatism is initiated at very shallow levels, as oceanic slabs entering trenches bend and are potentially infiltrated by seawater and as sedimentary sections begin their journey into forearcs resulting in extensive physical compaction, fluid expulsion, and diagenetic alteration. Studies of forearc fluid geochemistry (e.g., in accretionary complexes and, for the Marianas margin, in serpentinite seamounts) track this shallow-level metasomatic alteration, whereas high- and ultrahigh-pressure metamorphic suites provide records of fluid generation and flow, and related metasomatism to depths approaching those beneath volcanic fronts (and, in a smaller number of cases, depths beyond those beneath arcs). Uncertainty remains regarding the geochemical influence of strongly mechanically mixed zones along the slab-mantle interface (i.e., in the “subduction channel”), represented by mélange zones in many metamorphic suites. Experimental studies predict dramatic change in the physicochemical properties, and metasomatic capabilities, of subduction-zone “fluids,” as a function of depth. However, the studies of metamorphic suites have yet to document results of this change and, toward this goal, further work is warranted on UHP suites representing subduction-zone depths of 100 km or greater. Work on higher-P suites is also necessary in order to test the generally accepted hypothesis that subduction-zone metamorphism serves as a geochemical “filter” altering the compositions of deeply subducting rocks that then contribute to arc magmatism and the geochemical heterogeneity of the deeper mantle sampled by ocean-island basalts.