Concentration ratios of aromatic compounds, such as naphthalenes, phenanthrenes, and dibenzothiophenes are at present attracting increasing attention as maturity indicators in sedimentary rocks and crude oils. Generally, these indicators rely either on an increase with maturity in the degree of alkylation of a given parent compound or a shift in the isomer distribution of alkyl-aromatic homologues towards thermally more stable isomers. A combination of both concepts in the Methylphenanthrene Index (MPI) resulted in an excellent maturity parameter, as demonstrated for rock extracts by an improved correlation with mean vitrinite reflectance (Rm) between 0.65 and 1.4%, i.e., in the zone of oil formation. In cases that Rm measurements were ambiguous, this empirical relationship has proved useful in the calculation of theoretical vitrinite reflectance (Re) values from the MPI. On the other hand, for potential petroleum source rocks reaching maturity at relatively low temperatures, the influence of the organic matter type on the aromatic distributions cannot be neglected. Consequently, organic facies effects should be taken into account in the interpretation of aromatic distributions of such source rocks and related crude oils. Since the impact of the organic facies is strongly reduced with increasing maturity, aromatic maturity parameters are particularly useful in the maturity evaluation of post-mature crude oils and condensates. In contrast to biological markers, substantially higher concentrations of the key alkyl-aromatics persist at elevated maturation levels. Thus, aromatic maturity parameters should be considered as versatile tools in organic maturation studies, and can complement established maturity indicators based on steranes and terpanes.