Recent development in STM measurements allows us to look into the detailed mechanism of catalytic reactions at metal surfaces on the atomic level. The topics for the surface elementary steps such as adsorption, dissociation, diffusion, surface reaction and reconstruction are reviewed in conjunction with surface heterogeneity and molecular interaction, which are essential for the understanding of catalytic reactions at surfaces. As for the heterogeneous character, dissociation of molecules at step edges, surface reactions at the perimeter of island, and precursor state of molecular adsorption are visualized and proved by STM. The other aspects of the topics are related to molecular or atomic diffusion, existence of hot atom, and chemical reconstruction. These insights obtained by STM will provide the basis of a new type of theory for catalytic reactions.