The fundamental process of oceanic subduction is explored in this chapter. It discusses subduction at global scale, including how subducted lithosphere sinks into the mantle, to the transition zone between the upper and lower mantle, and sometimes all the way to the core, creating subduction slab graveyards. These graveyards are linked to velocity variations near the mantle-core boundary. The subduction zone itself is explored at crustal levels from the slab to the top. This includes the bending, fracturing and serpentinization of the oceanic slab itself, the volcanic arc, the forearc basin. The chapter discusses hydration of the downgoing slab in the context of fluid circulation and melt formation, and how melting produces shallower magmatism. It also discusses the geochemical signature of arc magmatism and the associated back-arc heat flow. It also explores trench advance and trench retreat and consequences for subduction zone processes. Oblique subduction is covered together with strain partitioning into strike-slip and orthogonal motion along convergent plate boundaries. Examples from Central America, the Sunda trench (Sumatra), and from other location around the Pacific Ocean. Toward the end of the chapter, the complicated subject of subduction initiation is discussed.