Infrared spectroscopy is probably the most powerful single technique available to qualitatively identify organic materials and to determine molecular structure. Mass spectrometry gives the molecular weight and formula, and nuclear magnetic resonance the number and type of protons, but only infrared indicates in a direct manner the presence of key functional groups. A match between the infrared spectra of an unknown sample and a reference sample is a simple, and almost positive, method of identification. Currently, over 130,000 reference spectra are available, most of which are available in indexed form. Spectroscopic theory will be explained in terms of very simple molecules, and then extensions will be briefly indicated to use with construction composites.