Recent advances in gas-phase ion chemistry are discussed. These include theoretical and experimental approaches to ion structure determinations, ion spectroscopy and photodissociation, unimolecular dissociation reactions of polyatomic cations and ion-molecule reactions. Considerable progress in many of these areas of research has come through the experimental use of tandem (MS/MS) mass spectrometers. From the theoretical viewpoint,ab initiocalculations have demonstrated predictive capabilities for ion structure determinations and statistical theories—quasiequilibrium theory (QET) and variational transition state theory (VTST)—are still of great value in treating the dynamics of ionic systems.