Recent legislation directed at reducing sulfur levels in petroleum-based fuels has led to numerous studies of commercial hydrotreating, the process whereby sulfur, nitrogen, and oxygen are removed from organic compounds in petroleum feedstocks. The goal of this overview is to highlight organometallic and clean surface model studies that offer realistic ways of understanding details of hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and hydrodeoxygenation (HDO) reactions as they occur on heterogeneous hydrotreating catalysts. (C) 1997 Elsevier Science Ltd.