A major change that has occurred in high-power transmitting valves during the last decade is the introduction of the thoriated tungsten filament or cathode. The paper describes the technology of these cathodes and some of the valve characteristics resulting from the change-over from a pure tungsten cathode. Details are given of the various properties of thoriated tungsten cathodes resulting from the carburization process which is necessary to give stable emission. The performance is given of six typical types of transmitting valves used both in the sound and television services of the B.B.C. These are typical of many other types of large valves with thoriated cathodes which differ only in minor aspects from those described. Detailed performance results are given, together with a study of the economies of thoriated cathodes in practice. In considering the installation of these valves in place of the valves using bright tungsten cathodes, consideration has been given to the economic aspects of installation. Illustrations of the six types of valves described in the paper and survivor curves both for tungsten and thoriated tungsten valves are given, together with the curves showing loss of programme due to valve failure.It seems clear from the results that not only do thoriated tungsten filaments or cathodes give economy in operation, but the lives in many cases are considerably extended.