Field emitting cathodes, which are central to the new technology of vacuum microelectronics, are emergingas novel sources of electrons. Development of such cathodes is proceeding at many centres around the world,with applications in microwave devices and displays. Recent work, as part of a collaboration between theUniversity of Strathclyde' and GEC-Marconi Ltd.', has involved the design and development of a field emissionarray (FEA) cathode for a vacuum diode millimetre-wave gyrotron. Initially this will be a low power, Ã¢â‚¬â€ lkWmm-wave output, long pulse, > 500, system. There are several advantages of using such cathodes as anelectron beam source in gyrotrons including increased beam quality, low velocity spread and a well-definedand controllable spatial beam distribution. Further advantages lie in the ability to operate such cathodes gated,long pulse or DC. Treating the cathode and grid as a simple RC network predicts possible gating frequencies> 1MHz. Previous cold cathode gyrotrons have been operated' at full power ( 6MW mm-wave output) at arepetition frequency of 330Hz that was limited by the power supply. Utilising FEA's promises ultimate repetition frequency three orders of magnitude beyond this, as the gating frequency is not related to the switching speed of the HT power supply.