Upon burial, organic matter in sedimentary rock undergoes numerous compositional changes that are dictated initially by microbial agencies and later mainly by thermal stress. This continuum of processes is termed thermal maturation and is divided into three consecutive stages: diagenesis (R₀ < 0.5%), catagenesis (0.5% < R₀ < 2.0%), and metagenesis (2.0% < R₀ < 4.0%). Kerogen, the major global precursor of petroleum, consists of selectively preserved, resistant, cellular organic materials (algal, pollen, spores, and leaf cuticle) and the degraded residues of less resistant biological organic matter (amorphous material) in variable proportions. Kerogen formation is complete by the end of dia enesis. The mode of kerogen formation exerts a strong influence on its structure and bulk composition, and hence on oil- and gas-generating characteristics, during catagenesis. Sulfur-rich type II kerogen, occurring in carbonate--evaporite source rocks, can generate oil at low levels of thermal stress. Low sulfur type II kerogen requires more thermal energy to generate oil, and type I and type III kerogens still more. High-wax oils appear to be generated from both wax ester and biopolymeric precursors, the first of which generates at an early stage of catagenesis and the other throughout catagenesis. The base of the oil window (occurring at R₀ < 1.3%) appears to be controlled by the depletion of long chain components in the case of some terrigenous kerogens and by oil stabi ity in the case of marine kerogens. In the latter part of catagenesis, all source rocks contain strongly enhanced proportions of hydrocarbon gases (wet gas). Throughout metagenesis, source rock kerogens are strongly depleted in hydrogen and generated gases consist of methane (dry gas) and sometimes hydrogen sulfide or nitrogen. Nevertheless, original oil potential can sometimes be recognized. Thermal maturity level can be monitored using a wide array of parameters. Steranes, hopanes, and other biological markers respond sensitively to thermal stress. Because their pseudokinetic parameters are known, these reactions are frequently used in numerical simulation to calibrate thermal history.