This paper aims at elucidating the microstructural origin of strain hardening in open-cell metal foams. We have developed a multiscale model that allows to study the development of plasticity at two length scales: (i) the development of plastic zones inside individual struts (microscopic scale) and (ii) the formation of plastic localization bands at the scale of the cellular architecture (mesoscopic scale). We address how plasticity at both scales contribute to the macroscopic yielding and strain hardening of cellular metals. One of the important results is that, in contrast to strain hardening in dense metals, strain hardening in cellular metals consist of a synergistic contribution of two sources: (i) strain hardening of the solid material (microscopic scale) and (ii) geometric hardening due to strut reorientation (mesoscopic scale). We show that the synergy of the two leads to an enhanced macroscopic hardening capacity. Our results are in qualitative agreement with experimental studies and elucidate the microstructural origin of plastic hardening in this class of materials. (C) 2010 Elsevier Ltd. All rights reserved.