The application of first-principle quantum chemical calculations toward analyzing and understanding heterogeneously catalyzed C-X bond activation is reviewed. More specifically, two industrially relevant systems, methane activation and thiophene desulfurization are discussed. Density functional theoretical (DFT) calculations are used to examine C-H activation of acetate a precursor to acetate decarboxylation and C-S activation of thiophene. Both systems require a sufficient cluster size model for the catalyst in order to predict energetic information, and hence model periodic trends. C-H activation of acetate closely mimics the activation of methane. The reaction path is characterized by a late transition state with respect to the C-H bond stretch. There is considerable M-H and M-C bond formation. The predicted barrier is +115 kJ/mol. Thiophene hydrodesulfurization which occurs via eta 1 adsorption of thiophene, is initially activated by hydrogenating thiophene to 2,5 dihydrothiophene (DHT). DHT activation proceeds via a metal atom insertion into the C-S bond. The formation of an additional M-S bond (two-fold sulfur) stabilizes the metallocycle intermediate and lowers the barrier of DHT C-S bond breaking from +250 to +150 kJ/mol.