In this feature article we review some of the striking results we have obtained with scanning tunneling microscopy (STM) on two single-crystal metal surfaces, Cu(110) and Ag(110), related to several general principles of surface reactions. While the "classic" surface science techniques are invaluable in providing supplementary information, STM has opened our eyes to the reality of the way in which surfaces participate in reactions. Simple adsorbates can form several nanometer-sized phases. Mixtures of reactants form cooperative structures that are far from homogeneous. Surface reactivity is site-specific and anisotropic. Step defects can have specific influences on reactivity. Reaction intermediates can inhibit the reactivity of the adsorbed reactants. The surface metal atoms rearrange in a manner far more complex than anticipated by the flexible surface model. We find that the reaction intermediates incorporate a stoichiometric number of metal atoms in the adsorbed layer, which leads to a significant amount of mass transport of metal atoms over the surface, altering the surface landscape. The alterable topography manifests itself as the formation of nanometer-sized islands or pits with a specific coverage. Such reaction-induced nano-restructuring has been observed for a number of reaction systems. The behavior of the surface in reactions is remarkably complex.