Intra-oceanic island arc-trench systems represent a locus of crustal thickening through the addition of mantle-derived magmas which form the volcanoes and subjacent stratiform plutons. The crust of immature island arcs and adjacent back-arc basins is oceanic in chemistry in that it is dominated by mafic (e.g., tholeiitic) magmas. A magma sequence is recognized in many arc systems which progress from boninite to arc-tholeiite to calc-alkaline and, in some arcs, to shoshonites. The adjacent back-arc basins are formed of MORB-like tholeiite. Arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled. Extension of the arc system may be associated with seaward retreat or " roll back" of the trench axis. Alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore-arc and accretion of fragments from the incoming sea floor. The Izu-Bonin, Mariana, Tonga, Yap and Palau trench-arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces. Crustal extension causes both fore-arc subsidence and opening of back-arc basins. Island arc volcanic chains and back-arc basins are new crust formed largely of fractional melts of the mantle. The evolution of an arc system may stop when blocking of the subduction zone, or changes in relative plate motion, causes overthrusting of back-arc lithosphere on to former volcanic arcs. Many ophiolites preserve remnants of arc system crust and upper mantle.