This paper presents a critical examination and analysis of classical and recently proposed models for transport phenomena in polymer electrolyte membranes. Key experimental observations related to membrane conductivity, membrane hydration, and sorption isotherms are first reviewed. Proton transport mechanisms in bulk water, and the influence of the membrane phase on these mechanisms, are examined. Finally, various formulations and underlying assumptions to account for macroscopic transport are reviewed, and an analysis of the binary friction model (BFM) and dusty fluid model (DFM) is performed to resolve an outstanding formulation issue. It is shown that the BFM provides a physically consistent modeling framework and implicitly accounts for viscous transport (i.e., Schloegl equation), whereas the dusty fluid model erroneously accounts twice for viscous transport. In Part II we apply the BFM framework to develop a general transport model for perfluorosulfonic acid membranes. (c) 2005 The Electrochemical Society.