Essential features of rotational spectra are developed on the basis of the quantum mechanics of molecular rotation. The effects of the harmonic vibrations of the nuclei are considered on the molecular rotation. Permanent electric dipole moments are required for the observation of pure rotational transitions. The Stark effect is used to determine the electric dipole moment. Nuclear quadrupole splittings give information on the electronic charge distribution in molecules. Large‐amplitude internal motions in molecules lead to splittings of rotational transitions. The splittings determine the potential barriers of the internal motions.