  Formed at an early stage of gas-phase ion-molecule chemistry, hydrides --
molecules containing a heavy element covalently bonded to one or more hydrogen
atoms -- play an important role in interstellar chemistry as they are the
progenitors of larger and more complex species in the interstellar medium. In
recent years, the careful analysis of the spectral signatures of hydrides have
led to their use as tracers of different constituents, and phases of the
interstellar medium and in particular the more diffuse environments. Diffuse
clouds form an essential link in the stellar gas life-cycle as they connect
both the late and early stages of stellar evolution. As a result, diffuse
clouds are continuously replenished by material which makes them reservoirs for
heavy elements and hence ideal laboratories for the study of astrochemistry.
This review will journey through a renaissance of hydride observations
detailing puzzling hydride discoveries and chemical mysteries with special
focus carbon-bearing hydrides to demonstrate the big impact of these small
molecules and ending with remarks on the future of their studies.
