“Rotational spectroscopy” is the technique that measures the interaction of electromagnetic radiation with the rotational movements of a molecular system. The observation of transitions between rotational quantum levels requires that the system under study has rotational freedom – typically, molecules, ions or radicals in the gas phase. This limitation does not allow the widespread application of rotational spectroscopy as an analytical technique, as in the case of other spectroscopic techniques that admit samples in condensed phase (i.e., liquids or solids). However, it is a historically important technique in the rigorous determination of molecular geometries and also has a very relevant contribution in astrochemistry: it is through rotational spectroscopy that several molecules (and molecular ions) are identified in space, using large radio telescopes such as ALMA (Atacama Large Millimeter Array).