A personal review of the use of molecular beam systems for the investigations of adsorption and surface reactions is given. This includes the determination of absolute adsorptivities (sticking probabilities), and reaction kinetics in transient, pseudo steady-state and steady-state modes. In the latter type of experiment true catalytic turnover can be measured. Molecular beam systems can rightly be classified as catalytic reactors when used for this kind of work. They are in a special class of reactors which is ideal for determining catalytic kinetics, namely, single collision reactors. Examples of the application of this kind of technology will be given, including CO oxidation on Rh and selective methanol oxidation on Cu.