Abstract: Vacuum electron devices have wide applications in medicine, communication and navigation. The cathode is the core of the device, and impregnants are the indispensable component of the cathode since they could effectively improve the cathode emission property. In this paper, the development history, the preparation methods of the impregnants, active composition in the impregnant, the formation mechanism, and the role of the active substance in the cathode performance have been reviewed, and the impregnants have been divided into several kinds of alkaline earth metal aluminate. The first generation of barium aluminate impregnant marked the initial application of alkaline earth metal aluminate in dispenser cathodes, providing low emission capability. The second generation of barium aluminates, including 5BaO·3CaO·2Al2O3(532), 4BaO·1CaO·1Al2O3(411) and 6BaO·1CaO·2Al2O3(612), have been received intensive studies and practical applications. The third generation of multicomponent impregnant, such as scandium containing aluminates and strontium containing aluminates, have been developed and were taken as the most promising impregnants to meet the demand for high frequency and high-power vacuum electronic devices due to their high emission property. Finally, an outlook on the development of alkaline earth metal aluminates is presented.