With the increasingly complex electromagnetic environment， for a receiver， especially in non-cooperative condition， identification and analysis of emitter sources are regarded as an important method to obtain electronic intelligence. Emitter identification refers to the process of extracting and analyzing characteristics of emitter signals intercepted by the reconnaissance equipment. The category and type of emitters is made. As a momentous direction in the electronic counter measures（ECM）area， emitter identification has a significant meaning as well as a wide range of applications in radio safety and spectrum resources management. Based on the analysis of a great deal of domestic and foreign literature，this article starts with the main characteristics of emitter identification in simplicity， then elaborates the classification and main methods of emitter identification. The actualities of emitter identification are introduced. Emitter data sets are categorized. This review ends up with possible future development trends in emitter identification.