The Multiple Beam Klystron (MBK) is really exploited in the USSR only. In recent years, there has arisen interest in the other countries. At present, the MBK with fundamental mode cavities are predominate, recently there is renewed interest in more complicated schemes. The advantages of MBK for the designer are low individual beam perveance, together with low total electron stream resistance and high total power. That helps obtain the high electronic and circuit efficiency and contributes to high gain, all of which are very important for broadening of instantaneous operating band. Of course, the MBK creates some difficulties for the designer. They are the non-uniformity of cavity rf field distribution and the transverse contributions of focusing magnetic field. There are remedies, but they lead to some complications. The broadening of instantaneous operating band requires tight drift channels packing, and also klystron life depends strongly on cathode-state-of-the-art. In spite of these difficulties, we have very successful low-voltage MBKs for various radar and communication applications in the frequency ranges from 1 to 10 GHz, with high peak and average power, and bandwidths up to 5–10&percent;. In Russia today, single-beam klystrons are limited primarily to the mm-range. ©1999 American Institute of Physics.