The pivotal component in a high-power microwave (HPM) weapon is the device in which the HPM pulses are generated. The microwave source selected for a particular application determines the properties of the emitted radiation and poses more or less restrictive requirements on the pulsed-power system used for generating the highvoltage pulse powering the radiation source. This report gives an overview of narrow-band microwave devices based on the generation of coherent electromagnetic radiation from electron beams travelling in vacuum. The vircator and the MILO are treated in more detail with simple analytical descriptions. The information provided in this report will form a cornerstone in future detailed and dedicated investigations of microwave sources for HPM-applications.