"The Molecular Universe"

Observations of astrophysical molecules, carefully interpreted, provide unique information of general astronomical interest. Three recent examples involving interstellar hydride molecules will be presented. I will discuss how (1) measurements of the molecular hydrogen ortho-to-para ratio, made with the Infrared Space Observatory (ISO) and the Spitzer Space Telescope, provide a fossil record of the thermal history of the interstellar medium; (2) determinations of the interstellar water and hydroxyl abundances, carried out with the Submillimeter Wave Astronomy Satellite (SWAS) and the Arecibo radio observatory, provide chemical evidence for a warm gas component within cold molecular gas clouds; and (3) observations of hydrogen fluoride, performed with ISO and planned for the Herschel Space Observatory, promise a unique probe of molecular gas within predominantly atomic clouds.