Far-infrared or Terahertz spectroscopy (THz) is a technique that identifies chemicals based on the interaction of molecules with electromagnetic radiation in the far-infrared or terahertz region (20-400 cm-¹). Molecular absorbances of far-infrared light cause molecular rotations (similar to microwave radiation) and can be classified by chemical functional groups. The resulting THz spectrum produced is characteristic for a given molecule. THz spectra of pure materials can be searched against reference spectral databases, although an expert spectroscopist should examine data from any potential unknown mixtures. One strength THz spectroscopy offers is the ability to differentiate polymorphic differences readily compared to other optical spectroscopy (MIR, NIR, UV-Vis) methods. This lends the technique to analysis of pharmaceuticals, consumer products or any other material that has multiple polymorph forms.  The technique is most often used in the analysis of inorganic and organometallic species, although condensed phase biological samples have also been investigated with THz spectroscopy.   Typically samples are condensed phase materials, although some specialized systems are capable of analyzing gases in gas cells. Bench top systems are most common.  Instruments are available from a number of manufacturers.