The spectroscopic investigation of the vibrational energy levels of molecules. In the infrared region of the electromagnetic spectrum vibrational transitions are accompanied by rotational transitions. Infrared spectra of molecules are series of bands, with each band being associated with a vibrational transition and every line in that band being associated with a rotational transition that accompanies the vibrational transition.

Some features of vibrational spectra can be analysed by regarding the vibrations as simple harmonic motion, but a realistic account of molecular vibrations requires that anharmonicity is taken into account.

A diatomic molecule can only have a vibrational–rotational spectrum if it has a permanent dipole moment. A polyatomic molecule can only have a vibrational–rotational spectrum if the normal modes of vibration cause the molecule to have an oscillating dipole moment.