Understanding the formation of a bond between a metallic surface and an adsorbate is critical to the field of heterogeneous catalysis. In principle, the bond (free) energies of all intermediates and transition states of a process fully determines the rate, but in most cases the number of such bond energies is so large that it becomes very difficult to single out why a given catalyst surface is the optimal catalyst for the reaction studied. Scaling relations have provided a method to define a few bond energies as descriptors of the catalytic rate. It has been found quite generally that, for a given surface structure, the bond energies of intermediates and transition states scale linearly with a few bond energies, and once these relations have been found, the full kinetics can be described as a function of the descriptors. These scaling relations result in the "volcano" relationships between rates/selectivities and descriptors that are used extensively in heterogeneous catalysis. It provides a theoretical basis for the heuristic Sabatier principle, which suggests that the optimal catalyst is one with an optimal bond energies for relevant intermediate(s). The scaling relations show which intermediate(s) are relevant and the optimal value(s) for the binding energy of these intermediate(s).