The industrial hydrodenitrogenation (HDN) process entails the removal of nitrogen from heteroaromatic nitrogen compounds in petroleum feedstocks under very high temperatures (400–600°C) and high pressures (200 bar) of hydrogen gas. In order to understand the various steps associated with this complicated HDN reaction, organometallic chemists have attempted to define each important reaction utilizing a homogeneous catalysis approach. Therefore, studies on the modeling of the HDN process, with an emphasis on the synthetic and mechanistic aspects of the homogeneous catalytic hydrogenation of model petroleum, heteroaromatic nitrogen compounds with organometallic complexes as catalysts, at low temperature and low pressures of hydrogen gas, will be reviewed in this article. A comparison of the regioselectivities under various hydrogenation conditions for these compounds will be discussed for a wide variety of transition-metal complexes with regard to the role of substrate binding at the metal ion center. The polynuclear heteroaromatic nitrogen compounds appear to hydrogenate more readily, under similar reaction conditions, in comparison with model petroleum compounds with mononuclear ring systems, e.g., pyridine. The relative rates of hydrogenation of a variety of heteroaromatic nitrogen compounds as well as compounds that inhibit and enhance selective hydrogenation of the nitrogen-containing ring will also be a focus of this review. A relatively new spectroscopic technique, high pressure nuclear magnetic resonance spectroscopy, will be shown to be a powerful tool to elucidate the mechanisms of the regioselective hydrogenation of polynuclear heteroaromatic nitrogen compounds. Finally, selected studies by other contributors to this nascent field will also be placed in perspective.