The rising deployment numbers of electric propulsion systems and the increased price of noble gas propellants have created the demand for alternative propellants. Iodine is a very promising candidate, which has already been successfully demonstrated with a variety of thruster types. The main challenge remains to design an iodine compatible neutralizer for those thruster types requiring neutralization. This review first gives an overview of the most common neutralizer principles and categorizes them according to their efficiency and current generation. Special consideration is given to plasma bridge hot cathodes, as they allow the broadest scaling of the supplied current levels. Different emitter types and materials are then discussed based on their resistance to iodine corrosion. In the final section, the experimental results of neutralizers tested with iodine are compared and the encountered difficulties are reviewed. A summary presents the best candidates, based on the maximum neutralization current and the technological readiness level. The recommended neutralizer technologies for ultra-low currents are filament cathodes, for low currents radio-frequency-cathodes and for high currents plasma bridge hot cathodes. Filament cathodes have already been flight proven in an iodine propulsion system and radio-frequency-cathodes have been tested successfully in a laboratory environment. While possible in theory, a successful long-term test of an iodine-fueled plasma bridge hot cathode has not been achieved so far.