A quiet revolution has been taking place in the geophysical community. Assisted by better measurements and faster processing, seismic data have become an essential tool for reservoir management. Using seismic to monitor the behavior of reservoirs in production is not a new concept, but it has taken some time to evolve from a theoretical principle to a practical necessity. By developing the ability to perform highly accurate time-lapse seismic surveys, production geophysicists have been able to observe and quantify changes in the seismic data that can only be the result of fluid movement within the reservoir. This provides a heretofore unavailable macro-image of the reservoir's behavior over time that can be used to calibrate sophisticated numerical simulators, long used by reservoir engineers to predict performance.