Earthquake Early Warning is a new technology which is based on high density seismic network． In this paper，the authors discussed some of the basic concepts，key technologies， early-warning results and early-warning deployment，and its role in the aspect of earthquake prevention and disaster mitigation． This paper points out the existing problems of Earthquake Early Warning and the research directions． The paper puts forward for the first time the concepts of early warning capacity，early warning invalid region，early warning benefit area，and early warning response time． Earthquake Early Warning capacity is constrained by early warning system． Besides the network density，early warning model，automatic processing system，data transmission mode，and information distribution system，the actual Earthquake Early Warning capacity is related to the nature of earthquake rupture． The paper emphasizes that the earthquake warning deployment is a complex social engineering，and it requires the participation of the whole society．