While the physical chemistry of gas-phase ions has its roots in traditional mass spectrometry, the growth of the field during the past two decades can be attributed to the development and application of new experimental and theoretical techniques. Among these are guided ion beams for investigating ion-molecule reactions at very low translational energies, ion chromatography in which an ion’s electronic state or geometrical structure can be determined and selected, pulsed field ionization/zero kinetic energy photoelectron spectroscopy with +/-0.1 meV resolution for determining ionization energies and ion vibrational frequencies, and multiphoton ionization and photoelectron-photoion coincidence methods for state selecting ions in uni;and bimolecular ionic reactions. The results from all of these studies have been greatly enhanced by concomitant advances in ab initio molecular orbital methods which provide heats of formation, structures, and vibrational frequencies of stable ion structures as well as transition states. This information is now widely used in interpreting collisional and unimolecular dissociation rates with the statistical theory, Rice-Ramsperger-Kassel-Marcus/ quasiequilibrium theory. Near quantitative agreement between theory and experiment is now the norm. The confidence with which ion chemistry is now understood for small systems has permitted the application of these methods to molecular systems of interest in condensed phases and in organometallic and biological chemistry.