We present experiments that explore the role of crystallographic steps in regulating the propagation of surface chemical processes. Using scanning tunneling microscopy as the primary structural probe, we track the evolution of step structures on clean metal surfaces, under chemical adsorption, and under reaction conditions. Substantial step motions are observed during these processes on vicinal Ag(110) and Al(111) substrates, even at room temperature, revealing these steps as efficient adatom sources and sinks. The interplay between these step motions and molecular events is further investigated. Atomically resolved STM measurements of adlayer structures, in conjunction with infrared spectroscopic characterization of the adsorbates, reveal the lateral distribution of adsorbates with respect to the step edge, and their chemical speciation. For the oxidation and hydrogenation reactions that we have investigated on these metallic substrates, steps serve two principal roles: They act as efficient adatom sources for the formation of adsorbate-substrate complexes. They introduce domain boundaries that facilitate mass flow in both adsorption and reaction.