Surface sensitive spectroscopies, and more recently scanning tunnelling microscopy, have provided a new insight into the dynamics of oxygen chemisorption at metal Surfaces. Studies at low temperatures. using probe molecules. provided the first evidence for the role of metastable "hot" oxygen transients and molecular complexes in opening up low-energy pathways in oxidation catalysis. Progress over the past 15 years is considered, emphasizing the limitations of the classical approach in discussions Of the Mechanisms Of surface reactions and how concepts developed through surface spectroscopics could be sustained at the atom-resolved level.