The paper dealing with aging of polymer electrolyte membrane fuel cells is divided in two parts. We present first general comments on the significant issue in an attempt to introduce the topic and to provide basic elements of methodology for investigation: aging mechanisms, reversible or irreversible damage, tests for investigation, and the case of cyclic operations. Then, two examples of cycled operations are presented and discussed for the case of long period cycles-as for day-night applications-operated to single cells. The current density is cycled between two levels in the first example, whereas the second example concerns cycling of relative humidity of inlet air in two fuel cells of different sizes.