Abstract: Vacuum electron devices have wide applications in aerospace, radar communications, research equipment, and medical devices. They occupy an irreplaceable position in high-frequency, high-power microwave devices. As the most widely used electron-emitting material in vacuum electronic devices, the electron-emitting ability and lifetime of dispenser cathodes directly determine the device’s performance. Dispenser cathodes are mainly composed of a porous tungsten matrix and an embedded impregnant. The impregnants are indispensable components of the dispenser cathode, as they effectively improve its emission properties. An in-depth analysis and summary of the development history of impregnants is crucial for developing novel dispenser cathodes. In this paper, we review the development history, preparation methods of the impregnants, the active composition within the impregnants, the formation mechanism, and the role of the active substances in cathode performance. According to the evolution of their composition, impregnants have been classified into three types of alkaline earth metal aluminates. The first generation of impregnants is BaO–Al₂O₃ binary aluminates. This marks the initial application of alkaline earth metal aluminates in dispenser cathodes, providing low emission capability and establishing dispenser cathodes as an essential branch of cathode materials. The second generation of impregnants consists of BaO–CaO–Al₂O₃ ternary aluminates, developed by adding CaO to the first generation of impregnants. Over more than half a century, several typical impregnants have emerged, including 532 aluminates (5BaO–3CaO–2Al₂O₃), 411 aluminates (4BaO–1CaO–1Al₂O₃), and 612 aluminates (6BaO–1CaO–2Al₂O₃). Dispenser cathodes using second-generation impregnants exhibit excellent performance and have been extensively studied and practically applied. The third generation of multicomponent impregnants, such as scandium-containing aluminates and strontium-containing impregnants, has been developed. These are considered the most promising impregnants to meet the demands for high-frequency and high-power vacuum electronic devices due to their high emission properties. The third generation of impregnants is still at the laboratory stage owing to disadvantages such as higher melting points and poor emission uniformity. Furthermore, the electron emission properties of all current dispenser cathodes are summarized and compared, providing a valuable reference for researchers. Additionally, the importance of phase diagrams for alkaline earth metal aluminates is addressed by analyzing and summarizing the BaO–Al₂O₃ binary and BaO–CaO–Al₂O₃ ternary phase diagrams. Data on the structure of various impregnants and the phase diagrams of binary and ternary alkaline earth metal aluminates clarify the contribution of different impregnants to cathode performance. Finally, an outlook on the development of alkaline earth aluminates for dispenser cathodes is presented, including crystal structure analysis, stability adjustments, and preparation process optimization.