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For many years the presence of waxes in crude oils has been associated with organic matter derived from terrigenous sources. High temperature gas chromatography (HTGC) has been used to establish the ubiquitous presence of high molecular weight (HMW) hydrocarbons, extending as high as C120, in crude oils. HMW hydrocarbons (>C40+) have been observed in crude oils derived from terrigenous, lacustrine, and marine source materials. Modified extraction techniques have also led to the detection of HMW hydrocarbons in source rock extracts. The fractions of HMW hydrocarbons are complex mixtures of long-chain aliphatic and alkyl-aromatic hydrocarbons, and can represent up to 8% of a whole oil. The carbon preference index (CPI) of HMW alkylcyclopentanes in the C42–C46range provides useful information on the characteristics of the original depositional environment. Age, maturity, and degree of biodegradation do not appear to affect the CPI of alkylcyclopentanes used for evaluating depositional environments. Differences in the distributions of the HMW hydrocarbons between the source rock extracts and the crude oils in the reservoirs may reflect changes occurring due to migration.