This project develops real-time, high-precision earthquake location algorithms for use in operational seismic monitoring procedures. The new software, DD-RT, which uses waveform based double-difference concepts, harnesses the vast digital archives of past seismicity and enables the immediate evaluation of new events with respect to the spatio-temporal pattern of past earthquakes. This work has societal and economic impacts in the immediate evaluation and mitigation of seismic hazards, and the potential for forecasting future events. The DD-RT software was developed to improve seismic monitoring capabilities in Northern California, and it now routinely computes high-precision locations of earthquakes recorded by the Northern California Seismic Network. These results are continuously made available at http://ddrt.ldeo.columbia.edu.