Surface chemistry, like other branches of physical chemistry, historically developed through macrosopic studies. These included measurements of adsorption-desorption equilibria (adsorption isotherms), spreading of monomolecular films, surface dissociation of diatomic molecules, and kinetic studies of desorption, sticking, and catalytic oxidation of CO and H-2. Models of surface structures were proposed by Langmuir and Taylor on the bases of experimental findings. Molecular level studies of surface chemistry were delayed as compared to other fields of physical chemistry until the late 1950s as instrumentation to detect properties of the very small number of surface atoms (10(15) cm(-2)) in the presence of a large number of bulk atoms (10(22) cm(-3)) were not available. At present we have over 65 techniques (photon, electron, molecule, and ion scattering, and scanning probes) that can investigate composition, atomic and electronic structures, and the dynamics of their motion with less than or equal to1% of a monolayer sensitivity. Key results include: quantitative determinations of surface segregation of constituents that minimize surface free energy, discovery of clean surface reconstruction and adsorbate induced restructuring, and the uniquely high chemical activity of rough surfaces and defects (steps and kinks). In situ molecular studies during surface reaction reveal the need for restructuring of metal surface atoms by a highly mobile strongly adsorbed overlayer to maintain catalytic activity: additives that inhibit mobility on the surface poison chemical reactivity. New techniques permitting molecular surface studies at high pressures and at solid-liquid interfaces greatly accelerated the developments of molecular surface chemistry and permitted in situ studies of complex surface chemical phenomena: catalytic reactions, electrode surface chemistry, and polymer surfaces. As always, further developments of techniques control the rate of progress of molecular surface chemistry.