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The objective of the project is to develop and improve upon three-dimensional (3-D) seismic velocity models of the Earth and to utilize such models for improving the locations of events recorded at regional and teleseismic distances. Our previous experiments performed using 3-D models of varying resolution show that the accuracy of event location with respect to ground truth does not necessarily improve with an increasing number of free parameters in a model. One reason for this may be the lack of waveform data used for constructing recent high-resolution models, since the maximum sensitivity of seismic travel times to structure is in the middle to lower mantle. In this paper we develop a new joint, P and S velocity model parameterized in terms of radial and horizontal cubic splines, using a combination of direct and differential travel times and surface wave phase measurements. The accuracy of the travel time data has been improved by relocating the events using a previously existing 3-D mantle P wave model. The cubic spline parameterization conveniently allows more detailed models to be inserted within it for use of regional phases in event location. Our preliminary model shows substantial differences in compressional velocity in the uppermost mantle compared with previous models. In conjunction with the model development, we are collecting new reference events with accurate locations, among them earthquakes on mid-ocean ridges and transforms, using a constrained inversion technique., Proceedings of the Annual DoD/DOE Seismic Research Symposium (22nd): Planning for Verification of and Compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) held in New Orleans, Louisiana on 13-15 September 2000. U.S. Government or Federal Rights. The original document contains color images.