The activation of O-2 on metal surfaces is a critical process for heterogeneous catalysis and materials oxidation. Fundamental studies of well-defined metal surfaces using a variety of techniques have given crucial insight into the mechanisms, energetics, and dynamics of O-2 adsorption and dissociation. Here, trends in the activation of O-2 on transition metal surfaces are discussed, and various O-2 adsorption states are described in terms of both electronic structure and geometry. The mechanism and dynamics of O-2 dissociation are also reviewed, including the importance of the spin transition. The reactivity of O-2 and O toward reactant molecules is also briefly discussed in the context of catalysis. The reactivity of a surface toward O-2 generally correlates with the adsorption strength of O, the tendency to oxidize, and the heat of formation of the oxide. Periodic trends can be rationalized in terms of attractive and repulsive interactions with the d-band, such that inert metals tend to feature a full d band that is low energy and has a large spatial overlap with adsorbate states. More open surfaces or undercoordinated defect sites can be much more reactive than close-packed surfaces. Reactions between O and other species tend to be more prevalent than reactions between O-2 and other species, particularly on more reactive surfaces.