This work reviews the progress in the area of high current density electron emitters, specifically for use in high frequency and high-power microwave vacuum electron devices. The review is divided into two subsections: 1) thermionic cathodes and 2) field emission cathodes. The thermionic cathode section includes the discussion on the M-type, MM-type, Scandate and the CPD cathodes. CPD cathode promises better usability in a practical device as compared to the Scandate cathodes which are limited by emission nonuniformity, poor resistance to poisoning/ion bombardment and low life. With the increasing demand for miniaturized and high power VEDs in the near future, the development of high current density field emission cathodes is of immediate interest to the VED community. The Spindt, CNT, and graphene-based emitters are the potential candidates in the field emitters section. Graphene-based film cathodes, when compared to the Mo/Si FEAs and the CNTs, offer larger emission area as well as high current carrying capability. This review includes the evolution of the emission capabilities of the emitters, state-of-the-art performances, and possible future developments. Index Terms-Dispenser cathodes, field emitter arrays (FEAs), vacuum electron devices (VEDs). I. INTRODUCTION R ECENTLY, the development of a compact terahertz (THz) microwave vacuum electron devices (VEDs) have received considerable attention for a variety of application areas such as communication, remote sensing, security, and medical imaging. These devices offer high-energy conversion efficiency, thermal robustness, and radiation hardness compared to their solid-state counterpart [1]. With the reduction in the transverse dimension of the device at high frequency, the need for a copious, rugged, and high emission density electron source is probably one of the greatest technological challenges in realizing a compact high frequency VEDs.