One of the factors that affect the polymer electrolyte fuel cell (PEFC) performance is the mechanical compression inside the stack. Although this compression plays an important role to ensure efficient gas sealing along with optimised electrical and thermal conductivities during PEFCs operation, excessive mechanical pressure may worsen the fuel cell performance through reducing the porosity and transport ability of PEFC components. In the last few years, intensive research studies have focused on the gas diffusion layers (GDLs) due to the strong relation of their compressibility with the performance of the PEFCs. A few review papers investigating the compression effect on individual fuel cell components have already been published. However, the evaluation of this effect on an operating PEFC has rarely been reviewed. This paper covers a comprehensive overview of the studies that have focused on the relationship between mechanical compression and the observed performance of a PEFC operating in real life conditions (in-situ). In this study, the effects of GDL properties with respect to the applied mechanical compression and the operating conditions are investigated. Much more work in literature has focused on GDL properties. Thus, an extended discussion is dedicated to this cell element in this paper.