Asphaltenes raise trouble in petroleum processing. During the hydroprocessing of heavy feedstock, asphaltenes limit the efficiency of conversion and refining, act as coke precursors leading to catalyst deactivation. The effect of asphaltenes and their solubility in oil on the formation of solids have been extensively studied in the last decades. In this paper, the state of the an is reviewed, covering the changes in the chemical structure of asphaltenes upon hydrotreatment, leading to the reduction in product stability. As a result, asphaftenes might precipitate, causing coke deposition on the catalyst and the formation of deposits in the refinery equipment. This is the main reason for the limitation of conversion resulting in the reduction in the product yield. (c) 2005 Elsevier B.V. All rights reserved.