Compounds significant to the petroleum chemist concerned with petroleum exploration are those hydrocarbons possessing biological marker characteristics, that is, possessing intact steroid, terpenoid, and isoprenoid skeletons. These hydrocarbons are so closely related to the compounds occurring in the living organism from which petroleum was formed that they are capable of yielding very specific information regarding source, maturation, migration, and biodegradation of petroleum. Examples how source shales can be related to petroleum reservoirs using computerized gas chromatography/mass spectrometry, (GC/MS) are shown. The terpane GC/MS multiple array processor (MAP) approach can be used to differentiate source shales of different degrees of maturity and, thus, to determine source rock quality. Heavily biodegraded oils can be characterized and correlated by GC/MS fragmentograms of steranes and terpanes. Ratios of specific biomarkers obtained by quantitation from GC/MS data are used to differentiate oils of different degrees of migration. Examples from the exploration arena worldwide are given to illustrate these applications.