We summarize the available knowledge of subduction gained from geology, petrology, and seismology and consider the advantages and drawbacks of each method. Regional and local seismic tomography reveals complex systems of fluid and melt migration at different depths above subducting slabs. The physicochemical evolution of the magma storage system maintaining active volcanism is modeled with reference to a wealth of geological and geophysical data. Subduction-related processes are discussed by examples of active (Kamchatka and Japan arcs) and ancient (Kokchetav metamorphic complex) subduction zones. Comprehensive geological and geophysical studies in Kamchatka and Japan prove valid the leading role of andesite magma in subduction of oceanic crust and, on the other hand, show that modeling independent migration paths of melts and fluids is problematic. The case study of Kamchatka provides more insights into melting in intermediate magma reservoirs at the depths about 50-80 and 30. km and highlights the significance of shallow magma sources at the pre-eruption stage. The Kokchetav metamorphics, which are exhumed suprasubduction rocks, offer an exceptional opportunity to estimate directly the compositions and ages of subduction-related melts.