The stability of (Mg,Fe)SiO3 perovskite in the deep lower mantle has long been uncertain. Recently, a phase transition from perovskite to postperovskite was discovered through a significant change in the X-ray diffraction pattern at high-pressure and high-temperature conditions corresponding to the core-mantle boundary region. This phase transition was also confirmed by first-principles calculations. These suggest that (Mg,Fe)SiO3 postperovskite is the predominant mineral in the lowermost mantle called the D″ layer, and phase transition from perovskite to postperovskite has significant geophysical implications for its nature and dynamics. The postperovskite phase may account for the large seismic anomalies observed in the D″ region, such as the D″ discontinuity, S wave anisotropy, and anticorrelation between the anomalies in S wave and bulk sound velocities. In addition, this phase transition is a strongly exothermic reaction. It destabilizes the thermal boundary layer at the base of the mantle and should promote the formation of high-temperature upwelling plumes.