Earthquake early warning (EEW) is notification of an earthquake's occurrence in real time, that is, after the earthquake has begun. EEW was first successfully implemented in the UrEDAS (Urgent Earthquake Detection and Alarm System) system for Japan Railways in the 1980s. EEW has safely stopped high-speed trains in Japan and provided warnings in Mexico City. More recently, research has identified characteristics of the seismic P wave that permit warning from a single on-site detector. A fully functional EEW system requires not only seismological detectors and processing, but also decision-making and communications apparatus designed in accordance with social science findings.