Water is transported by subducting slabs into the transition zone and lower mantle. Important water carriers to the deep mantle may be serpentine, chlorite, phase A, and superhydrous phase B in peridotite; zoisite, lawsonite, and phengite in basalt; and topaz-OH and phase Egg in sediments. Phase D, stable in peridotite, and the δ-AlOOH phase in the sedimentary component may transport water to at least 1200–1500 km depth. Phase relations in hydrous peridotite show that the phase boundaries of olivine to wadsleyite and ringwoodite to Mg-perovskite + ferropericlase phase transitions shift to lower and higher pressures, respectively. Thus, elevation of the 410 km discontinuity and depression of the 650 km discontinuity in subduction zones may be partially affected by water. Water may also control the separation of the basaltic layer of the slab near the 650 km discontinuity. The density crossover that occurs under dry conditions between peridotite and basalt components of slab near 650 km disappears under hydrous conditions due to a significant shift to lower pressures of post-garnet transformation in basalt. Due to high water solubility in wadsleyite and ringwoodite, the transition zone may be a water reservoir in Earth's interior. Recent data show that the transition zone at least locally may contain 0.2–1.5 wt% H₂O. In addition, the transition zone may serve as a water absorber for the water circulation system of the mantle. The upper mantle appears to be largely degassed through the action of mid-ocean-ridge and hotspot volcanism. The water storage capacity of the lower mantle as well as hydrogen storage potential of the core are still uncertain. There are several potential dehydration sites in the mantle that may control water circulation through plate tectonics: the mantle wedge above subduction slabs, a region above the 410 km discontinuity, the top of the lower mantle, and the deep lower mantle.