The need to develop an electrolytic membrane for an efficient, environmentally sound fuel cell has led to intense interest in proton conducting polymers in general and Nafion in particular. While it does not appear very likely that Nafion itself will ultimately prove to be the best choice-of material, it may be considered as a prototype membrane material. Initial interest focused on Nafion＇s potential use in a hydrogen fuel cell, in which case its conductivity in the presence of water is important, and so extensive studies of the electrical properties of Nafion at various levels of humidity were carried out. Two distinct regimes were identified, one at lower water contents and the other at high water contents. The possible conduction mechanisms associated with these regimes will be discussed. In addition, studies carried out at high pressure yielded activation volumes which provide further clues as to the conduction mechanisms involved. More recently, interest in Nafion as a membrane : material in methanol fuel cells has prompted investigation of its electrical properties in the presence of methanol alone and of methanol/water mixtures. It is clear that not only is Nafion an excellent proton conductor but it also exhibits significant methanol transport. This represents a serious crossover problem for fuel cell applications and it is important to be able to characterize the mechanisms involved.