This review article was constructed around the first Algerian-French congress aimed on emerging materials which was held at Tamanrasset by the end of February 2003. The aim of this review is to point out that a lot of work has been done in heterogeneous catalysis to better understand the active sites responsible for the catalytic reactivity. Most of these researches were performed under reducible atmosphere, on metallic catalysts, to improve our knowledge about hydrocarbon reforming catalysts. Starting from this base which was recalled through various classes of important studies such as: (i) the dilution of the active sites, (ii) the use of bimetallics, (iii) the use of well-crystallised surfaces and (iv) the influence of the metal-support interactions; a development and an opening is made on the three-way catalysis and the DeNO(x) reactions. The objectives being to point out the very important influence of the experimental conditions and of the gas phase compositions which may induce very strong surface modifications of the initial metallic aggregates. Moreover, it will depend on the reactions, i.e. isomerisation, oxidation, reduction where the active site may also be composed of, in addition to the metallic crystallites, the participation of the oxygen of the support. A tentative for a general interpretation of the observed results is given by the use of the variations of the local density of states and of the "d" band centre energy. (C) 2003 Elsevier B.V. All rights reserved.