Publisher Summary For paramagnetic species on electrode surfaces or inside the active materials of electrodes, in-situ electron spin resonance (ESR) is a unique technique that can reveal important information about these species, such as their identity, quantity, molecular motions, and interactions with microenvironment. The use of porous carbon electrodes has proved to be an effective solution to improve the ESR signals for paramagnetic species on electrode surfaces. Besides detecting and identifying the surface species, in-situ ESR is able to provide useful information about the molecular motions and possible potential-dependent ligand field effects. When electron spins are densely distributed in three-dimensional surface films or in the active materials of the electrodes, in-situ ESR studies are technically less difficult than those for adsorbed molecules because of the better signal/noise (S/N) ratio. In view of the variety of conducting polymers and lithiated carbons, the methods described in the chapter may be used for more samples to check the generality of the conclusion reached.