X-ray computed tomography (XCT), a non-destructive technique, is proposed for three-dimensional, multi-length scale characterization of complex failure modes in fuel cell electrodes. Similar to medical CT scanners, laboratory XCT enables low-intensity X-ray imaging of a specimen at different incident angles followed by reconstruction into threedimensional views. Fuel cell materials are compatible with this technique as they are sufficiently transparent to X-rays. In this thesis, electrode failures are analyzed by comparative tomography data sets for conditioned beginning of test (BOT) and degraded end of test (EOT) membrane electrode assemblies subjected to cathode degradation. Cracks and thickness of the cathode catalyst layer (CCL) are analyzed at the micro length scale, followed by a complementary nano length scale analysis of the fine porous structure. Additionally, a novel image processing based technique is developed for nano scale segregation of pore, ionomer, and Pt/C dominated voxels in the degraded CCL. The results of this work reveal several failure modes of catalyst layers including but not limited to carbon corrosion, Pt agglomeration, and Pt migration. In summary, XCT based multi-length scale analysis enables detailed information needed for comprehensive understanding of the complex failure modes observed in fuel cell electrodes.