[en] The continuing need for accelerators of increasingly sophisticated characteristics requires a great variety of rf sources to provide the energy used by these accelerators. The present status of gridded tubes will be reviewed briefly. Klystrons, which are used in most large accelerators, will be considered in some detail. Their limitations in power and frequency will be reviewed and a study of their reliability will be presented. Improvements in their efficiency will be discussed, and trends for the future will be considered. Some limitations caused by material problems will become evident and a review of some of the potential improvements in that area will be considered. Finally, new types of rf sources based on rotating electron beams which are now in the development stage will be described and compared to existing tube types