A great deal of research has been conducted on the infrared spectra of pure molecules at low temperatures. Only a limited amount of information can be acquired on molecules in the liquid, amorphous, and crystalline states. The theories describing the liquid and amorphous are not yet completely adequate, whereas the theories describing the gaseous and crystalline states appear to be well established. The value of lowering the temperature of molecules appears to be that of simplifying their spectra by increasing the order of the system.