Electrospinning is gaining popularity as a convenient and robust technique for fabricating non-woven mats of sub-micron diameter polymer fibers. Although not as well studied, the technique can also be used to prepare particle/polymer fiber networks with high intra- and inter-fiber porosity.  Such fibrous networks can be used as porous electrodes in fuel cells and batteries, where high interfacial electrode area, i.e., the accessibility of electrode material within a 3-D anode or cathode architecture, is of prime importance. In this talk, a review of recent experimental work on nanofiber mat architectures for hydrogen/air fuel cell cathodes and lithium battery anodes and cathodes will be presented, where the particle loading in the fibers is very high. The fuel cell work is focused on fibers containing carbon supported Pt or Pt-alloy powders with various perfluorosulfonic acid polymer based catalyst binders. The fuel cell power output using such a cathode is very high for ultra-low Pt loadings.  Additionally, carbon corrosion of nanofiber cathodes, as determined from voltage cycling experiments, can be minimized by proper adjustment of the hydrophilic/hydrophobic properties of the catalyst binder. For Li battery applications, nanofiber anodes have been prepared and evaluated, where the fibers are composed of either TiO₂ and carbon powders with poly(acrylic acid) binder or carbon powder and poly(vinylidene fluoride) (PVDF). Here, the volumetric capacity of a fiber anode at charge/discharge rates of 1C-3C are considerably higher than those measured with a conventional slurry anode. Electrospinning can also be used to prepare ultra-thick anodes with high areal capacities, e.g., 2.5 and 1.3 mAh/cm² at charge/discharge rates of 1C and 2C, respectively. Similarly, Li battery nanofiber cathodes with LiCoO₂ and carbon powders in a PVDF binder have been fabricated and evaluated at various areal capacities and C-rates.   Experimental details for both fuel cell and battery electrodes will be presented in this talk, including the procedures for electrospinning fibers, the methods for characterizing the fiber mats, and the properties and performance of the particle/polymer mats in a given application.