Hydrogen is known to affect elastic and anelastic properties of mantle rocks and minerals. Hydrogen dissolution in minerals notably alters the properties of transition zone phases, which may accommodate very high water contents. Moreover, even small amounts of water can induce partial melting in certain mantle regions and modify seismic wave velocities and attenuation. Progress in seismic imaging of the mantle—particularly the mapping of seismic attenuation and velocities—has improved constraints on local hydrous melt content in the upper mantle, and evidence exists for partial melt-bearing layers above and below the transition zone owing to dehydration reactions induced by upward or downward flow of mantle material. Further observational and modeling studies are needed to more fully understand the influence of hydrous melting on the global water cycle, mantle viscosity, and large-scale geodynamics.