The existence of complex molecules in the harsh environment of interstellar diffuse and dense clouds presents the challenging problem as to their mode of synthesis. Currently, this is believed to be predominantly via many parallel and sequential gas phase ion-neutral reactions which build-up large molecular ions which are then neutralized to give the observed molecules. The obJ.ctive of this paper is to review the current ideas of the most important ion-chemical routes to the observed molecules and to discuss the most recent laboratory data which have supplemented and extended earlier ideas. To this end, the physical conditions within the various types of interstellar clouds and their chemical compositions are first described. The basic ion-chemical models and some of the more important ionic reactions involved in them are then described as they variously apply to both dense and diffuse clouds. Reference is then made to the data obtained recently for the rate coefficients and product ion distributions for a large number of relevant exoergic binary ion-molecule and ion-atom reactions. Attention is then directed towards the process of ion-molecule radiative association which for some time was thought to occur only between C+ions and H atoms and H2molecules, but which is now thought to occur in many more ion-molecule interactions, including many involving minority neutral species. The very interesting process of isotope exchange in near-thermoneutral ion-molecule reactions is then considered since it has been invoked to explain the apparent fractionation of rare isotopes into some of the interstellar molecules. Finally, the various neutralization processes which are required to convert product ions into neutral molecules are discussed briefly, and the paper is ended with some concluding remarks.