Physical methods are increasingly applied to the solution of structural problems in the field of organic chemistry and the use of infrared spectroscopy is standard practice in most laboratories today. Fast recording infrared spectrometers became commercially available in about 1946. This comprehensive article contains the following topics as related to infrared spectra of natural products: methods, instruments, and sampling techniques; applications, compound comparison, structural analysis; steroids and terpenoids; studies of stereochemistry, axial and equatorial hydroxyl groups, bromo-ketones; and, application of infrared spectroscopy to the structure and configuration of long-chain polyenes, mycomycin, and carotenoids.