The redistribution of heat by fluid circulation in subducting igneous crust generates thermal anomalies that can affect the alteration of material both within a subduction zone and in the incoming plate prior to subduction. This hydrothermal circulation mines heat from subducted crust and transports it seaward, resulting in anomalously high temperatures in material seaward of the trench and anomalously low temperatures in the subduction zone. Anomalously high temperatures on the incoming plate are spatially limited; for example, on the Nankai margin of southern Japan, a zone of high temperatures is within ~30 km of the accretionary prism deformation front. The incoming plate (Shikoku Basin) undergoes the high-temperature anomaly for less than 2 million years; so the alteration of clay minerals in Shikoku basin sediments advances only slightly because of the thermal anomaly. In contrast, subducted material is cooled by hydrothermal circulation, and therefore alteration of subducted sediment and igneous rock is shifted farther landward (i.e., delayed); in the Cascadia and Nankai margins, this includes the seismically inferred locations of the basalt-to-eclogite transition in the subducting crust. In very hot margins, hydrothermal circulation cools the subducting slab and affects where, and if, subducting material may melt. In southern Chile, this cooling helps explain the lack of a basaltic melt signature in arc lavas despite the young subducting lithosphere. Finally, the cooling of the subducting slab via hydrothermal circulation shifts fluid sources from dehydration reactions farther landward, delays metamorphic reactions that tend to reduce permeability, and increases fluid viscosity. The responses to hydrothermal circulation in subducting crust are most pronounced in the hottest subduction zones, where the lateral heat exchange in the subducting basement aquifer is greatest.