Discovery of the perovskite to post-perovskite phase transition in MgSiO3, expected to occur for deep mantle conditions, was first announced in April 2004. This immediately stimulated numerous studies in experimental and theoretical mineral physics, seismology, and geodynamics evaluating the implications of a major lower mantle phase change. A resulting revolution in our understanding of the D? region in the lowermost mantle is well underway. This monograph presents the multidisciplinary advances to date ensuing from interpreting deep mantle seismological structures and dynamical processes in the context of the experimentally and theoretically determined properties of the post-perovskite phase change; the last silicate phase change likely to occur with increasing pressure in lowermost mantle rocks.