Petroleum biodegradation is the alteration of crude oil by living organisms (e.g. Milner et al., 1977; Connan, 1984; Palmer, 1993; Blanc and Connan, 1994). Numerous eubacteria, fungi, and possibly archaea evolved metabolic pathways to consume saturated and aromatic hydrocarbons. The existence of these pathways in ancient and diverse prokaryotic lineages provides indirect evidence that petroleum systems occurred during the early Precambrian Age. Petroleum biodegradation is primarily a hydrocarbon oxidation process, producing CO2 and partially oxidized species, such as organic acids. Heteroatomic compounds are less susceptible to biological attack and increase in relative abundance as biodegradation proceeds. This increase is caused primarily by enrichment of pre-existing petroleum NSO compounds due to selective biodegradation of other compounds, but a secondary cause is the direct production of heteroatomic compounds (mainly oxidized species) by microbes. Petroleum quality and net volume thus decrease with increasing biodegradation. API gravity decreases while non-hydrocarbon gases, viscosity, NSO compounds, and trace metals increase.