A newly fabricated polymer electrolyte membrane (PEM) fuel cell usually needs a so-called break-in/conditioning/incubation period to activate it and reach its best performance. Typically, during this activation period the cell performance increases gradually, and then reaches a plateau without further increase. Depending on the membrane electrode assemblies, this process can take hours and even days to complete, which consumes a considerable amount of hydrogen fuel, leading to a higher operating cost. To provide for accelerated conditioning techniques that can complete the process in a short time period, this paper reviews established conditioning protocols and reported methods to condition PEM single cells and stacks, in an attempt to summarize available information on PEM fuel cell conditioning and the underlying mechanisms. Various techniques are arranged into two categories: on-line conditioning and off-line conditioning. For each technique, the experimental procedure and outcomes are outlined. Finally, weaknesses of the currently used conditioning techniques are indicated and further research efforts are proposed. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.