ISO, the Infrared Space Observatory is capable of measuring spectra across most of the mid-infrared, the region from 4000 to 500 cm−1 (2.5-20 μm). Of these 3500 cm−1, about 670 have been inaccessible to astronomers due to strong atmospheric absorptions and about 750 cm−1 require an airborne platform, making access limited. ISO will provide high quality spectra in the 19% of the celestial mid-infrared which has never been studied and dramatically increase the number of spectra in the 21% with limited access. Thus ISO has the potential to nearly double our knowledge of the mid-IR spectroscopic properties of the cosmos. This knowledge, in turn, will revolutionize our understanding of its chemical make-up because of the mid-infrared's powerful compositional diagnostic capabilities.