This study aims to elucidate the fundamental mechanisms of extensional stress-induced foaming phenomena by in situ observation of polypropylene foaming processes using a view-cell system. The foaming processes of polypropylene blown with supercritical carbon dioxide have been observed under static conditions as well as dynamic conditions where extensional strains were applied. A bubble growth-induced cell nucleation phenomenon, where existing cells triggered the generation of cells around them, was observed under static conditions due to the presence of crystals. In the dynamic cases, the applied extensional stresses caused foaming to occur in a more dispersed manner. The bubble growth-induced cell nucleation phenomenon was also observed, but it occurred at a later stage and the effect was less dominant. These fundamental understandings will provide guidance to improve processing strategies of plastic foaming processes. (C) 2013 Elsevier B.V. All rights reserved.