Proton exchange membrane (PEM) based technologies (fuel cells and electrolysers) offer promising sustainable power generation and storage solutions for a diverse range of stationary and mobile applications. Unitised regenerative fuel cell (URFC) is an electrochemical cell that can operate both as a fuel cell (FC) and an electrolyser (E). However, for a widespread commercialisation, further improvements are required that address the durability, performance, and cost limitations. One of the main challenging components in developing URFCs is the gas diffusion layer (GDL) as it plays different vital roles, some of which are paradoxical in FC and E-modes. Therefore, in this paper, the published research on GDL of PEM-URFCs as well as relevant studies on PEM fuel cells and electrolysers are critically reviewed. The materials and novel methods to address the corrosion in E-mode are discussed. This is followed by presenting and discussing different properties of GDLs affecting the performance in FC and E-modes: i.e. porosity, thickness, pore size, transport properties, thermal and electrical conductivity, and the GDL compressibility. Finally, the main modifications of the GDLs, such as hydrophobisation and microporous layer application, to improve the performance of a URFC are analysed and discussed. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.