The convergence of the Philippine Sea and the Eurasian plates in the Taiwan region led to the formation of a young collisional mountain between two subduction zones of nearly orthogonal polarities. The geological processes underlying the collision and its relation to subduction are the primary targets of TAIGER (Taiwan Integrated Geodynamic Research) project. Newly acquired passive and active sources data on land, supplemented by ocean bottom as well as permanent seismic network data, are used to derive a new 3-D tomographic velocity model. Using the 7.5km/sec contour as a marker for crustal deformation, two trend-parallel "roots," one under the Central Range with a maximum depth of 55km and the other under the Coastal Range at 40km, are found to extend from southern to central (∼24N) Taiwan. Between the two roots and lying approximately beneath the Longitudinal Valley, the 7.5km/sec contour rises to 25km depth. In the upper mantle, a high velocity zone, east-dipping and its upper surface coinciding with a Wadati-Benioff zone in the south (∼22.8N), becomes near vertical and ill-defined in the north (∼23.9N). The crustal deformation as defined by the "roots" and the along-trend variations of the upper mantle structures under Taiwan provide key information for the orogeny. With the thickening of crust to 55km or more processes such as eclogitization and delamination may come into play.