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Many specialized theoretical and experimental studies have been carried out on noble-gas plasma diodes in order to determine the operating characteristics of a particular mode. Even though the modes of cesium plasma diodes have been characterized rather completely by Bullis, no comparable summary has been developed for noble-gas systems. In this paper, the operation of hotcathode parallel-plane plasma diodes is reviewed and the characteristics of the anode-glow, ball-of-fire, Langmuir, temperature-limited, low-voltage arc, and tufted-anode modes are classified and discussed in terms of the volt-ampere characteristics for different load lines. The operation of noble-gas diodes is compared to that of cesium diodes and the interpretation of the volt-ampere characteristic is related to available experimental data. The visual characteristics of each mode are also displayed to aid in the interpretation of the behavior. The correlation between theory and a variety of experiments substantiates the contention that the limitation imposed on the diode current by the cathode is responsible for the previous confusion about the various modes of operation.