Gas diffusion layer (GDL) has a critical role in determining the performance of proton exchange membrane (PEM) fuel cells and electrolysers by facilitating uninterrupted and smooth gas and water transport to and from the electrochemically active areas of these cells. The design of GDLs has been evolved significantly during recent years with the key aim of improving their performance for delivering their role. This paper provides a comprehensive review of modifications introduced to improve the GDL performance to date. The majority of these modifications have been carried out on fuel cells and the GDL in electrolysers has been studied to a lesser extent. The main focus of these modifications was found to be on water management and reactant delivery improvement as per defined for the role of GDLs. This review covers in detail the modifications linked to GDL hydrophobicity, GDL porosity, and engineered perforated GDLs. The effect of introducing a microporous layer (MPL) to the GDL is also reviewed by considering key design aspects such as MPL thickness, pore size, porosity, hydrophobicity and hydrophilicity, and the role of cracks in MPL structure. This review comes up with the key research gaps that are yet to be addressed and the recommendation for future researches to be followed for GDL modifications (i.e. to achieve further improved performance in PEM fuel cells and electrolysers). (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.