Gaseous Molecular Ions provides an introduction to the subject. Organized in three sections, it treats in detail experimental methods used for the preparation of positive and negative ions and the study of their decomposition reactions. The instrumentation used in these methods is introduced, with an emphasis on the operating principles; basic theoretical concepts to describe the evolution of a molecular ion after its preparation are introduced; and negative molecular and cluster ions are covered. Electron attachement reactions for some prototype systems are presented. Particular emphasis is placed on how these processes change in proceeding from isolated molecules over molecular aggregates to condensed molecules.