AbstractHistorically the study of the spectra of diatomic molecular ions has been an important contributor to the development of molecular orbital theory. In view of the importance of molecular ions for radiation chemistry on the one hand and astrophysics (especially the study of the interstellar medium) on the other, we have started a program aimed at the discovery of new molecular ion spectra and at a better understanding of the structure of molecular ions. The various experimental techniques that have been used will be described. Particular attention has been focussed on the study of absorption spectra of molecular ions, since these absorption spectra would allow the study of those excited states which are subject to predissociation and therefore not easily observed in emission work. However, up to now only very few new spectra have been observed in absorption. On the other hand a number of new emission spectra of both diatomic and polyatomic molecular ions have been observed and the results about the structures of these ions will be discussed.