The crustal structure across the Tonga-Lau arc-back arc system from the Lau Ridge to the Pacific Plate (178degrees-170degreesW, 18degrees19degreesS) is modeled, using data from an 840-km-long air gun refraction line over 19 ocean bottom seismometers and one land station. The data reveal that the Pacific Plate crust is 5.5 km thick, with a velocity structure similar to that found at the present-day East Pacific Rise (EPR). Beneath Tonga Ridge, an intermediate velocity layer (6-7 km/s) is up to 7.5 km thick and has a velocity-depth distribution similar to andesitic rocks found in continental crust. The crust is abnormally thin (4 km) at the boundary between the Tonga Ridge and the Lau Basin. At the east end of Lau Basin, the crust is 5.5-6.5 km thick and resembles crust formed at the EPR except for a thicker sheeted-dike section (2-3 km) and thinner lower crust (2 km). The Lau Basin crust thickens to 7-8 km near the Central Lau Spreading Center (CLSC), mostly through thickening of the lower crust. The crust thickens again to 8.5-9.5 km at 50 km west of the CLSC, mostly through thickening of the midcrust. In the thick westernmost section, the crustal structure is uniform even though one part of this section formed through extension of arc-type crust while the rest was created at an oceanic spreading center. The relative homogeneity of these rocks suggests that their petrology may be dominated by postemplacement magmatic infilling from a mantle source west of the spreading center.