Polymer foams created using supercritical carbon dioxide as a processing solvent have garnered much interest in recent years. The properties of supercritical CO2 make it ideally suited to replace organic solvents such as chlorofluorocarbons that are being phased out for environmental reasons. A number of processes and products are already being used in industry, and we provide several examples of these developments. Yet, despite all that is now known about polymer/CO2 systems, a complete understanding of the foaming process, particularly nucleation, is lacking. In this article, we give an overview of the state of the art in polymer foaming with CO2 and highlight the challenges that remain to be overcome. (C) 2008 Elsevier B.V. All rights reserved.