The discovery of a large variety of molecules by radio astronomy has been a very strong motivation for the development of laboratory millimeter wave spectroscopy. Among them, the reactive species, neutral and/or ionic, have been early recognized as playing a very important role in the chemistry of the interstellar and circumstellar medium. While the laboratory spectroscopy of free radicals started relatively early, with the observation of the OH radical by the group of Townes (Dousmanis et al. 1955), the detection of molecular ions proved to be a much more difficult task, and the first millimeter line due to an ion was actually detected by radio astronomy (Buhl & Snyder 1970). It was called “U89.2” until it was tentatively attributed to HCO+ by Klemperer (1970) on the basis of both considerations on the chemistry of the interstellar medium, and of ab initio calculations for the prediction of the expected line frequency. This identification was later confirmed by more elaborated ab initio calculations (Wahlgren et al. 1973, Kraemers & Diercksen 1976), and by the observation of a transition attributed to H13CO+ (Snyder et al. 1976), but the definite confirmation was the observation of the same transition in a laboratory glow discharge by the group of Woods (Woods et al. 1975).