This paper presents a general overview of petroleum biorefining or petroleum refining using biotechnology. An explanation is offered as to why modern biotechnology, due to important advances that have occurred in genetic engineering, the study of extremophilic microorganisms, protein engineering and biocatalysis in nonaqueous media, can be a good alternative to develop new biocatalysts to be used in the refining of petroleum. An important refining objective is the desulfurization of fuels, and recent advances in research and development on fuel biodesulfurization (BDS) are discussed in this paper. Other possibilities in petroleum biorefining exist for the denitrogenation of fuels, removal of heavy metals, transformation of heavy crudes into light crudes as well as depolymerization of asphaltenes, and are also being addressed in this paper. All these alternatives are currently realized at the level of research and development and there are no industrial applications as yet. The development of biocatalysts for biorefining processes will depend on the involvement of experts in many areas, ranging from biotechnology to petroleum refining. (C) 2003 Elsevier Science B.V. All rights reserved.