Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer-Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal-carbon and metal-oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal. State-of-the-art quantum-chemical reaction data were used in a microkinetics simulations study to elucidate the different fundamental kinetic regimes underlying Fischer-Tropsch activity and selectivity. Based on the nature of the rate-controlling steps, three regimes were identified: I) monomer formation, II) chain-growth termination, and III) water formation.