Reforming over supported nickel catalysts has been used commercially for more than 40 years. In this paper four catalytic challenges for nickel steam reforming catalysts are discussed from both a practical and a fundamental point of view. The four challenges are activity, sulphur poisoning, carbon formation and sintering. A coherent microscopic picture of these important aspects of the working catalyst is emerging from classic studies and is now extended by in situ TEM and DFT calculations of atomic-scale relationships between structure and catalyst properties and new knowledge about sintering. (c) 2005 Elsevier B.V. All rights reserved.