AbstractA review is given of scanning tunneling microscopy (STM) studies of reactions between different chemical species on metal surfaces. Results presented for a variety of systems show that STM can give unique insight at the atomic level into the mechanistic details of surface reactions and decomposition of molecules. This is illustated with a number of examples of reactions occuring on single‐crystal metal surfaces, such as Ni(110), Cu(110), Ag (110), Rh (110), and Pt(111). The ability of the local STM probe to study the influence of defect sites on reactivity is highlighted, and similarities and differences in reactivity of different systems are discussed. Finally, the application of STM to enhance reactivity via tip interactions is addresse