Editorial The membrane-electrode assembly is the heart of the fuel cell, and materials science at the heart of MEA development. Besides standard perfluorosulfonic acid type systems, the development of viable alternative membranes still represents a significant challenge, and is an important topic attracting research investment world-wide. The search for reliable and durable membranes has an increasingly interdisciplinary character, with advances dependent on joint effort between polymer chemists and physicists, electrochemists and materials scientists, as well as specialists in the fields of interfacial and transport phenomena. In a PEMFC the membrane separates the anode and cathode reaction chambers in terms of electric isolation and reactant separation and provides proton conductivity. The operating conditions must be chosen in such a way that the membrane can retain mechani- cal integrity as well as provide proton conductivity. However increasing demands are being made on the properties of membrane materials since progress here can considerably simplify systems requirements. For automotive and station- ary applications, higher temperature of operation and low or no reactant gas humidification simplify thermal and humidification sub-systems but impose considerable constraints on the membrane. For automotive application, a materials option allowing fuel cell operation at sub-zero temperatures is also desirable. Many different issues must be addressed to achieve the ambitious goals. These comprise fundamental research and development of ionomers and polymer electrolyte membranes with an intensified research effort into the field of new types of membrane materials offering breakthrough solutions, ex situ and in situ characterisation, including the study of membrane degradation mechanisms that limit lifetime under dynamic operation conditions, a better understanding of proton transport mechanisms operating in water-assisted and quasi-dry or water-free proton transport, and the influence of membrane morphology on proton transport/transfer. This, and the following, issue of ªFuel Cells ± From Fundamentals to Systemsº are Topical Issues on Membranes for proton exchange membrane fuel cells. The articles in the present issue range from those devoted to synthesis of new ionomers, synthetic strategies to control membrane morphology and novel approaches to membrane processing, to several papers dedicated to study of membrane transport properties, membrane characterisation by X-ray and neutron scattering techniques, investigation of membrane degradation under accelerated ageing conditions and in situ characterisation in PEMFC and DMFC. The issue includes both reviews by internationally recognised leaders in given fields, and original research papers. I would like to most warmly thank all contributors to this Topical Issue who willingly embarked on the journey with me, and made the preparation of the Issue an enjoyable and rewarding experience. My thanks also to the efficient and most dependable Editorial Office, and to the Editor-in-Chief of ªFuel Cellsº who entrusted me with this task. Deborah Jones Montpellier, France March 2005 Deborah Jones Guest Editor EDITORIAL FUEL CELLS 2005, 5, No. 2 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 169