Interstellar hydrides - that is, molecules containing a single heavy element atom with one or more hydrogen atoms - were among the first molecules detected outside the solar system. They lie at the root of interstellar chemistry, being among the first species to form in initially atomic gas along with molecular hydrogen and its associated ions. Because the chemical pathways leading to the formation of interstellar hydrides are relatively simple, the analysis of the observed abundances is relatively straightforward and provides key information about the environments where hydrides are found. Recent years have seen rapid progress in our understanding of interstellar hydrides, thanks largely to far-infrared and submillimeter observations performed with the Herschel Space Observatory and SOFIA. An ongoing SOFIA legacy program, HyGAL, will provide a very significant expansion in the availability of relevant observational data. Carefully interpreted using sophisticated astrochemical models, HyGAL observations will unique information about the relationship between atomic and molecular gas in the interstellar medium; about the density of low energy cosmic-rays that ionize the gas; and about the dissipation of turbulence.
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