Unraveling the complexities of the upper mantle and transition zone continues to be a challenge for seismology, but the results play a crucial role in answering questions regarding the the composition, dynamics and evolution of the mantle. The substantial progress achieved in the last four years should continue as new analyses techniques are developed that take advantage of the rapidly expanding seismic datasets and the improvements in computer capabilities.
Acknowledgments. Guy Masters provided useful advice and comments. This research was partially supported by National Science Foundation grant EAR-93-15060.