Book excerpt: The symposium on Hydrotreatment and Hydrocracking of Oil Fractions aims to provide a global perspective and an inspection of the state-of-the-art of these processes. New American, European and Japanese environmental regulations call for advanced hydrotreatment processes for HDS and HDN for the removal of S- and Ni-components from oil fractions. These will alter the product slate of the oil refineries and the hydrocarbon composition of these products. Hydrocracking will play an important part in this shift. Adapting the operating conditions will not suffice to reach the desired product specifications and yields. Adequate catalysts will have to be developed. Powerful tools are now available for this, e.g. surface science techniques, molecular modeling and new types of reactors operated in a nonsteady mode. Another instrument in the improvement of hydrotreatment and hydrocracking units is the availability of more realistic kinetic models. These are based on a judicious insight into the reaction mechanism, also provided by the above-mentioned tools. Progress in the analytical techniques has allowed the reduction of the lumping of components in these kinetic models and first order kinetic equations are gradually replaced by equations accounting for the adsorption of the various components. More detailed and more realistic reactor models are now based on rigorous hydrodynamic models and their application has become possible through the rapidly increasing possibilities of computers.