Successful startup from subfreezing temperatures is a prerequisite for PEM fuel cell commercialization. In this paper, a comprehensive review of recent research progress on PEM fuel cell cold-start phenomena has been presented. Experimental studies are first briefly summarized. A transient multiphase multi-dimensional PEM fuel cell model, accommodating ice formation and temperature variation, is introduced. Numerical results from both isothermal operations and non-isothermal self-starts at subfreezing startup temperatures have been analyzed. The effects of many key parameters, including the water vapor concentration in the cathode gas channel, the initial membrane water content, the operating current density, and the startup cell temperature, on PEM fuel cell cold-start behaviors have been carefully examined to elucidate the fundamental physics of PEM fuel cell cold starts. Copyright (C) 2010 John Wiley & Sons, Ltd.