A total of one to ten ocean masses of water (including H2O, OH, H in gas, liquid, and solid phases) is probably stored in the Earth's crust-mantle system as a variety of forms and components. Of these, the liquid phase involves aqueous solution, hydrous melt, and supercritical fluid, which may change composition continuously from a diluted aqueous fluid to a silicate melt above the second critical endpoint. These fluids are termed “geofluids” in this review paper, which describes physicochemical conditions required for the presence of geofluids, their compositions, and related tectonic setting and geodynamic phenomena. On the basis of the maximum water content that can be stored in crust-mantle rocks, subduction zone processes are discussed in which slab-derived fluids that are enriched in solid components play important roles: viscosity-flow-thermal field and melting in the mantle wedge, and origin of deep-seated fluid such as Arima-type brine. The fluid flux in the arc crust may be captured by geophysical and geochemical observations, such as seismicity, seismic velocity, electrical conductivity, and helium isotopic ratios. Several specific phenomena such as migrating seismic events and localized crustal deformation may be attributed to such fluid fluxes from the depths, which have been induced by an elevated fluid pressure and a reduction of rock strength and viscosity.