During the past few decades, economic losses and human casualties due to natural disasters increased exponentially on our planet, mainly because of the increased density of population and industry in high hazard areas. Although the prediction of earthquakes is not practicable yet, the present technology allows a prompt identification of the onset of any dangerous seismic event before it hits an urban area. Earthquake early warning can provide an alert within a few tens of seconds in advance. This small lead time may be used to minimize property damage and loss of lives in metropolitan areas and to aid emergency response. The book provides information on the major EEW systems in operation and on the state of the art of the different blocks forming an EW system: the rapid detection and estimation of the earthquake’s focal parameters, the signal transmission, the engineering interface and the information reliability/false alarm problem.