Catalysis plays a key role in petroleum refining as most of the processes beyond the crude unit are catalytic. Since most straight-run fuels produced through fractional distillation do not meet the quality requirements for low-sulfur and higher-octane fuels, most of the crude unit fractions are further processed. Gasoline with anti-knocking characteristics is made with catalytic cracking of heavy hydrocarbons of selective catalysts and zeolites. Gasoline with low aromatic content (benzene/naphthalenes) comes from alkylation of light olefins with isobutane in the presence of hydrogen and metallic catalysts. The catalytic addition of hydrogen to atmospheric tower fractions results in the removal of Sulfur and Nitrogen and unsaturated compounds (olefins) via the Hydrotreating process. Increases in fuel octane numbers come from catalytic reforming and isomerization processes both of which are also catalytic in nature. Upgrading of heavy hydrocarbon streams through hydrocracking into lighter hydrocarbon molecules is catalytic.