A brief review is given of the role of molecular positive ions in interstellar chemistry. It is indicated how the simplest ions, which are formed by photoionization and cosmic-ray ionization, are converted to polyatomic ions by sequential gas-phase ionic reaction processes such as proton transfer, carbon insertion and radiative association. The importance of H3+ and CH3+ ions in the initial stages of molecular synthesis and the analogous roles of H2D+ and CH2D+ (which are formed in the reactions of H3+ and CH3+ with HD) in the production of deuterated molecules are stressed. Recent work is also mentioned concerning the possible routes to c-C3H2 the first cyclic molecule to be detected in interstellar clouds. The process of dissociative recombination of positive ions with electrons is also discussed, because it is commonly invoked as the final step in the destruction of polyatomic ions and in the formation of many of the observed neutral molecules in interstellar clouds, even though the products of such reactions are currently a matter for speculation. It is stressed how spectroscopic studies of the structures and the products of reactions of molecular ions can further advance understanding in the field of interstellar chemistry.