The start-up and varying load conditions encountered during a drive cycle introduce transient behavior in electrochemical performance as well as two phase flow and temperature of PEM fuel cells. The electrochemical transients have been extensively studied in literature, while the effects of two-phase flow transients and temperature transients have not been studied to any great detail. These two transients affect the electrochemical and mass transport performance in an automotive PEM fuel cell. Changes in load result in changes in flow rates and electrochemical reaction rates. These in turn result in changes in temperature field, two-phase flow patterns, area coverage ratio, and pressure drop. Transient operation leads to uneven power generation and loss in reliability. Transient changes in two-phase flow result in increased pressure drop, thereby increasing the auxiliary power consumption and the parasitic power losses. Thermal transients may result in flooding or dry-out of the GDL and membrane and lead to performance deterioration. Understanding transient behavior is thus critical for the reliable and predictable performance from the cell. A detailed review of the existing literature is presented along with the research needs in this area. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.