Some useful concepts for describing elementary reaction steps on surfaces will be reviewed, and illustrated with example applications to catalytic reactions. First, the characteristics which define a true "elementary step" will be considered. In general, the reaction order is well defined for true elementary steps, and their prefactors can be theoretically estimated with reasonable accuracy using transition state theory and simple concepts of statistical mechanics. Once the prefactor is known, a single absolute rate measurement provides the activation energy, which is thus obtained with good accuracy even when the prefactor used is in error by two orders of magnitude. The net reaction energy often can be measured independently. Examples of doing this with adsorption calorimetry will be reviewed. Recent developments promise to make calorimetric measurements of adsorption energies on single crystal surfaces more common, since they now can be applied to crystals as thick as obtainable by simple mechanical thinning (80 ïm). Once the net reaction energy is known for an elementary step, a measurement of its activation energy directly provides that for its reverse step and, through knowledge of statistical mechanics, its equilibrium constant. These kinetic and thermodynamic parameters are invaluable in examining or modeling the rates of complex reaction mechanisms, which in general turn out to depend on the kinetic and thermodynamic parameters of only a few of the many elementary steps involved. A simple method for determining which of the elementary steps are critical, based on the â?odegree of rate controlâ?, will be described. Work supported by DOE-OBES and NSF.