We are indebted to all authors of the seismic and geodynamic models for sharing their results and insights. We also want to thank the reviewers Rob van der Hilst and Hans-Peter Bunge as well as Bernhard Steinberger and James Kellogg for valuable suggestions on the manuscript. Richard O'Connell, Adam Dziewonski, and Göran Ekström were of great help, offering continuing support and encouragement. Funding for TWB was provided by a DAAD Doktorandenstipendium under the HSP-III program. Most figures were prepared with GMT by Wessel and Smith [1991].