In this paper, an attempt is made to overview some problems important for the development of high-power millimeter-wave drivers for future linear colliders. Since the microwave pulse duration required at high frequencies is much shorter than at low ones, two options seem possible. The first one is to develop 'moderate' power level, long-pulse tubes based on relatively reliable technology and then greatly compress these microwave pulses. The second one is to operate at much higher voltages and to directly generate very high-power pulses of the required length. Besides discussing pros and cons of these options, an overview of the methods of mode selection in oversized microwave circuits required for producing multi-megawatt power at millimeter wavelengths is presented. Also the issue of thermal limitations caused by microwave losses in circuit walls is discussed, and some scaling laws for the maximum power and pulse duration are given.
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