The complete catalytic cycle of the reaction of alkenes and alkynes to dienes by Grubbs ruthenium carbene complexes has been modeled at the B3LYP/LACV3P**+HB3LYP/LACVP* level of theory. The core structures of the substrates and the catalyst were used as models, namely, ethene, ethyne, hept-1-en-6-yne, (Me3P)(2)Cl2Ru=CH2, and [C2H4(NMe)(2)C](Me3P)Cl2Ru=CH2. Insight into the electronically most preferred mechanistic pathways was gained for both intermolecular as well as for intramolecular enyne metathesis. Alkene metathesis is predicted to proceed fast and reversible, while the insertion of the alkyne substrate is slower, irreversible, and kinetically regioselectivity determining. Ruthenacyclobut-2-ene structures do not exist as local minima in the catalytic cycle. Instead, vinylcarbene complexes are formed directly. The alkyne insertion step and the cycloreversion of 2-vinyl ruthenacyclobutanes feature comparable predicted overall barriers in intermolecular enyne metathesis. For intramolecular enyne metathesis, a noncyclic alkene fragment of the enyne substrate is first incorporated into the Grubbs catalyst by an alkene metathesis reaction. The subsequent insertion of the alkyne fragment then proceeds intramolecularly. Alkene association, cycloaddition, and cycloreversion to the diene product complex close the catalytic cycle. Rate enhancement by an ethene atmosphere (Mori's conditions) originates from a constantly higher overall alkene concentration that is necessary for the rate-limiting [2 + 2] cycloreversion step to the diene product complex.