The resultsof recently completed geological and geophysical studies of the tec tonically active protions of the Mariana Island arc system are described. Leg 60 of the Deep Sea Drilling Project, and the drill site selection cruises that preceeded Leg 60, provided the bulk of these new data. The Mariana forearc region consists of remnants of the Palau-Kyushu Ridge (rifted in the Oligocene) and the West Mariana Ridge (rifted in the late Miocene-early Pliocene), partially covered by modern arc sediments and volcanoes. Volcanism in the Mariana arc system seems to be continuous since Eocene, although periods of possible increased or decreased levels of volcanic activity can be interpreted from the drilling results. The hydrothermally active Mariana Trough has been opening for at least the past 5 MY at a half rate of 1.5-2.0 cm/yr, and is now spreading roughly east-west with a recognizable spreading center/transform fault geometry from an axial graben near 18 degree N latitude.