This is a critical review and compilation of the observed and predicted spectroscopic data on O2 and its ions O2-, O2+ and O22+ The ultraviolet, visible, infrared, Raman, microwave, and electron paramagnetic resonance spectra are included. Each electronic band system is discussed in detail, and tables of band origins and heads are given. The microwave and EPR data are also tabulated. Special subjects such as the dissociation energy of O2, perturbations, and predissociations are discussed. Potential energy curves are given, as well as f-values, Franck-Condon integrals, and other intensity factors. A summary table lists the molecular constants for all known electronic states of O2 and O2+ Electronic structure and theoretical calculations are also discussed.