This chapter covers infrared theory in a limited fashion. By understanding the theory, it is possible to see how the infrared spectra of complex molecules can be interpreted. It then becomes possible to see how the infrared spectrum of a material can be different depending on the state of the molecule (i.e., solid, liquid, or gas). This ultimately speaks to experimental realities associated with sample handling techniques. Quantum mechanics explains the presence of discrete energy levels. In order for a molecule to absorb infrared radiation, several conditions must be met. The frequency of the infrared source radiation must be identical to one of the transitions between the discrete energy levels of the molecule, and there must be a change in dipole moment. The position of a molecule in three dimensional space can be described in terms of the x, y, and z coordinate of each atom in the molecule. For large polyatomic molecules, numerous fundamental modes can exist.