Considerable progress has been made over the past two decades in spanning the well-known pressure and material gaps of heterogeneous catalysis. In particular, during the past decade several groups around the world have synthesized model catalysts consisting of metal clusters supported on planar oxide surfaces. These model catalysts are suitable for kinetic and spectroscopic investigations yet are amenable to study with scanning probe techniques at working temperatures and under realistic pressure conditions. This long-sought goal of observing a heterogeneous catalyst in an operating environment offers extraordinary new opportunities and poses significant new challenges for catalytic scientists of the 21st century. This article highlights some of these opportunities and challenges. (C) 2003 Elsevier Science (USA). All rights reserved.