Wide application of power electronic equipment has made the fault transient process of a power system more complicated and faster. This forces higher requirements on the “four characteristics” of relay protection. Therefore, the advantages of transient-based relay protection have become more prominent. First, the fault characteristics of the electronic power system are introduced. The fast adjustment and fragility of power electronic devices and their influence on the fault characteristics of a power system are analyzed in detail. Secondly, the application of transient quantities in line protection is comprehensively established, including traveling wave front protection, single-ended transient protection, etc. Then, new principles and methods of protection based on double-ended (multi-terminal) transients are analyzed. Finally, suggestions for research on transient-based protection are given. By rationally using the abundant transient information in the entire fault process, high-speed transient-based protection which could adaptively be matched to the fault is realized, and the “first line of defense” for power grid security can be fully established. This work is supported by National Key Research and Development of China (No. 2016YFB0900604), Science and Technology Project of State Grid Corporation of China (No. 52094017000W), and Key Program of National Natural Science Foundation of China (No. 51637005).