Our fundamental knowledge of interstellar grain composition has grown substantially during the past two decades thanks to significant advances in two areas: astronomical infrared spectroscopy and laboratory astrophysics. The opening of the mid-infrared, the spectral range from 4000-400 cm−1 (2.5-25 μm), to spectroscopic study has been critical to this progress because spectroscopy in this region reveals more about a material's molecular composition and structure than any other physical property.