Rate expressions for catalyzed reactions based on an understanding of the reaction mechanism are more likely to offer a good description of the kinetics over a wide range of reaction conditions than those developed empirically. This is particularly true when it is necessary to obtain an accurate description of the kinetics not only for steady-state but also non-steady-state conditions. Knowledge of the reaction mechanism and the rate coefficients for each step can also help identify which step, or steps, limit the activity of a catalyst, or control its product selectivity. While is often possible to postulate a mechanism that is consistent with the reaction kinetics, demonstrating that the proposed mechanism is physically correct is always a challenge. This talk will illustrate through a series of examples how different experimental theoretical methods can be used to identify the composition and structure of active centers, the composition and structure of reaction intermediates, and the connectivity of reactants, intermediates, and products. The examples presented will include oxidative dehydrogenation of alkanes to olefins, methane oxidation to formaldehyde, nitrous oxide decomposition, and olefin epoxidation.