Seismic P-wave velocity structure is estimated for the Puget Sound basin region using iterative, non-linear tomographic inversion based on finite-difference travel-time calculations. Local earthquakes and explosion arrival times are used to image the three-dimensional velocity structure to a depth of approximately 60 km. Our earthquake data set comprises approximately 3000 well-located digitally recorded earthquakes collected by the Pacific Northwest Seismograph Network (PNSN) over the 17 year period from 1980 to 1996. Additional constraint on velocity is provided by including travel-times, with known locations and origin times, from controlled-source seismic profiles and also by including model interpretations along one profile directly in the inversion. We find good correspondence between the prominent features imaged by this study and previous geological and geophysical interpretations. The method is effective in resolving high resolution structure by combining earthquake, explosion, and active source interpretations into a single velocity structure model.¿ 1997 American Geophysical Union