Publisher Summary This chapter discusses the basic principles of infrared and Raman spectroscopy. The chapter reveals that all light, including infrared, is classified as electromagnetic radiation and consists of alternating electric and magnetic fields. In quantum theory, radiation is emitted from a source in discrete units called photons. Photons of specific energy may be absorbed by a molecule resulting in a transfer of energy. The molecular motion that results from characteristic vibrations of molecules is described by the internal degrees of freedom resulting in the well-known 3n 6 and 3n 5 rules of thumb for vibrations for nonlinear and linear molecules, respectively. Vibrational spectroscopy relies heavily on the theoretical insight provided by quantum theory. Vibrational energy is not continuously variable but rather can only have certain discrete values. For energy to be transferred from the IR photon to the molecule via absorption, the molecular vibration must cause a change in the dipole moment of the molecule. The symmetry of a molecule, or the lack of it, will define what vibrations are Raman and IR active. Group theory is the mathematical discipline that applies symmetry concepts to vibrational spectroscopy and predicts which vibrations will be IR and Raman active. The standard method for calculating the fundamental vibrational frequencies and the normal vibrational coordinates is the Wilson GF matrix method. Accurate vibrational analysis requires optimizing the molecular structure and wave functions to obtain the minimum energy state of the molecule.