Heterogeneous catalysis is changing from an empirical art to an exact science. The various methods for the analysis of solids and surfaces, constantly refined by materials science and surface science, seem to be almost unlimited. The increasing availability of atomic resolution microscopy as well as synchrotron radiation allows the characterization of catalyst particles, surface structures, surface processes and surface intermediates. We have learned to determine the surface structure sensitivity of catalytic reactions. Thermodynamic and kinetic data of catalytic reactions are now determined routinely. Isotopic exchange and labeling experiments provide information about reactant-catalyst interactions. How much have we learned through these techniques about the nature or mechanism of heterogeneously catalyzed reactions? The following article attempts to summarize the progress and the problems encountered in mechanistic studies of CH bond formation and activation in a hydrogen atmosphere as an example for the present state of the understanding of reaction mechanisms in heterogeneous catalysis.