<p>Hydrogen isotopes (δD) in basaltic magmas record a wide range of processes controlling the distribution and cycling of water in the Earth’s interior, from planetary accretion and core formation to subduction recycling and shallow modification. This review synthesizes the hydrogen isotope systematics of glasses and olivine-hosted melt inclusions from mid-ocean ridges, ocean islands, back-arc basins, and arc settings, evaluating the processes that govern δD variability. Compilation of global datasets shows that regional δD variability reflects isotopically distinct source components: the depleted mantle is characterized by low despite heterogeneous δD values, while slab-derived fluids shift mantle domains toward higher δD values. The δD values of deeply subducted materials depend strongly on the extent of dehydration, with incompletely dehydrated slabs retaining elevated δD and extensively dehydrated residues developing anomalously low δD values. Extremely low δD values observed in some ocean island basalts further suggest the presence of highly D-depleted hydrogen in deep mantle sources, possibly involving core–mantle boundary exchange. In addition, shallow processes including magmatic degassing, secondary hydration, assimilation, and diffusive hydrogen exchange in melt inclusions can significantly modify primary δD values and produce trends that overlap with source-driven variability. Robust interpretation of mantle hydrogen isotopes therefore requires integration with complementary geochemical tracers such as radiogenic isotopes, noble gas systematics, and other volatile proxies. Comprehensively, these observations indicate that the Earth’s mantle is considerably heterogeneous in hydrogen isotope composition, reflecting the sensitivity of δD to processes operating across Earth’s history, from planetary accretion to present-day deep water cycle.</p>