The subduction model for trench-arc-back arc systems was deduced more or less as a logical consequence of sea-floor spreading. Many features in these systems, such as thrust-type inter-plate earthquakes and deep earthquakes along Wadati-Benioff zones, are readily explained by the subduction model. But, other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain. An approach to solve these problems may be to recognize the existence of the two basically different modes of subduction: one Chilean-type — causing a compressional stress regime in the arc and back arc regions; the other — Mariana-type — causing a tensional stress regime. Implications of the two types of the mode of subduction for some tectonic problems, such as vertical movement and sediment accretion at trenches are discussed, with special reference to the results of recent DSDP active margin drilling. In discussing the possible causes of the two modes, it is suggested that the large-scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction, but of some additional factors involving much greater energy, such as mantle flow associated with the motion of major plates. Finally, the possible great importance of various types of collision, accretion and erosion at subduction zones is emphasized.