How subduction begins and its consequences for global tectonics remain one of the essential outstanding problems of plate tectonics. Two different endmember mechanisms for subduction initiation have been hypothesized: spontaneous, and induced (or forced). Numerical models suggest that subduction initiation is induced by externally forced compression along a preexisting discontinuity in an oceanic plate such as a fracture zone or transform faults. However, it has been pointed out that spontaneous subduction must have occurred at some points in Earth's history to initiate plate tectonics, and recent numerical models demonstrated that lateral thermal/compositional buoyancy contrast along plate discontinuity or within lithosphere can cause spontaneous subduction initiation. Recent geological and geophysical surveys in the Izu-Bonin-Mariana fore-arc have revealed igneous processes in the initial stages of subduction. The oldest magmatism after subduction initiation generated MORB-like fore-arc basalts, which was associated with seafloor spreading caused by onset of sinking of slab into mantle. Then boninitic magmatism followed by tholeiitic to calc-alkaline arc lavas collectively makes up the extrusive sequence of the fore-arc crust. This magmatic evolution from initial basaltic magmatism to establishment of normal arc magmatism took several million years. Fore-arc stratigraphy observed in the Izu-Bonin-Mariana arc shares some of the key geologic and petrologic characteristics with many supra-subduction zone ophiolite, which implies that fore-arc crustal section produced in the initial stage of oceanic island arc formation could correspond to in-situ section of supra-subduction zone ophiolite prior to obduction. Recent ocean drilling projects targeting initial stage of the Izu-Bonin-Mariana arc inception revealed that subduction initiation to form the Izu-Bonin-Mariana arc took place spontaneously. The drilling results also revealed that the whole arc was established on the ocean crust produced associated with subduction initiation.