  Astronomical microwave spectroscopy has greatly broadened the inventory of
identifiable chemical species in diffuse molecular gas and is an increasingly
effective way to measure the abundances of polar molecules that may be
candidate diffuse interstellar band carriers. Here we review some recent
developments that hold new promise for chemical abundance determinations. We
summarize and categorize the molecular inventory that has accrued in the past
twenty years from microwave observations of diffuse clouds and we present
summary tables of the molecular abundances within various chemical families in
both dark and diffuse molecular gas.
