In petroleum geology and chemistry, cracking is the process whereby complex organic molecules (e.g. kerogens) are converted to simpler molecules (e.g. hydrocarbons) by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products is strongly dependent on the temperature and presence of any catalysts.

In thermal cracking, an atmosphere of elevated temperature and pressure is used. The first method, the Burton process, was invented by William M. Burton; the oil industry first using it to produce gasoline in 1913.

Catalytic cracking was used from around 1936, and uses a catalyst to aid the process of breaking down petroleum vapour to fractions. Typical catalysts include alumina, silica, zeolites, and various types of clay.

In catalytic cracking, less reactive and therefore more stabile and longer lived intermediate cations accumulate on the catalysts' active sites generating deposits of carbonaceous products generally (and in many cases inappropriately) known as coke. Such deposits need to be removed (usually by controlled burning) in order to restore catalyst activity. In thermal cracking an overall process of disproportionation can be observed, where "light", hydrogen-rich products are formed at the expense of heavier molecules which condense and are depleted of hydrogen.

Fluid catalytic cracking is now the most commonly used process and a modern USA due to the high demand for gasoline. The process was first used in around 1942, and employs a powdered catalyst. Initial process implementations were based on a reactor where the catalyst particles were suspended in an ascendant flow of feed hydrocarbons in a fluidized bed. In newer process variants, the contact time between the catalyst and the feed is greatly reduced in order to reduce the amount of coke deposited on the catalyst. The actual reactor is an ascendant_flow pipe called the "riser" in which pre_heated feed meets hot catalyst particles for just a few seconds before the catalyst is separated from the hydrocarbon using a cyclone, contacted with steam to strip off the remaining hydrocarbon and stop the reaction, and then transported into a fluidized-bed regenerator where air (or in some cases air plus oxygen) is used to burn off the coke to restore catalyst activity and also provide the necessary heat for the next reaction cycle, cracking being an endothermic reaction.

Steam cracking is a petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing the lighter alkenes, including ethylene and propylene.

Cracking processes allow the production of "light" products (such as LPG and gasoline) from heavier crude oil distillation fractions (such as gas oils and distillation residues). Fluid Catalytic Cracking (FCC for short) produces a high yield of petrol and LPG, while thermal cracking is currently used to "upgrade" very heavy fractions ("upgrading", "visbreaking"), or to produce light fractions or distillates, burner fuel and/or petroleum coke.

Two extremes of the thermal cracking in terms of product range are represented by the high-temperature process called "pyrolysis" (ca. 750-900°C) which produces valuable ethylene and other feeds for the petrochemical industry, and the milder-temperature "delayed coking" (ca. 500°C) which can produce, under the right conditions, valuable "needle coke", a highly crystalline petroleum coke used in the production of electrodes for the steel and aluminum industries.

More details about the cracking mechanism are provided in the alkane article.