Adsorbate interactions and reactions on metal surfaces have beeninvestigated using scanning tunneling microscopy. The mannersin which adsorbates perturb the surface electronic structure in theirvicinity are discussed. The effects these perturbations have on othermolecules are shown to be important in overlayer growth. Interactions of molecules with surface steps are addressed, and eachmolecule's electron affinity is shown to dictate its adsorption sitesat step edges. Standing waves emanating from steps are demonstrated to effect transient molecular adsorption up to 40 Å awayfrom the step edge. Halobenzene derivatives are used to demonstrate how the surface is important in aligning reactive intermediates.