A time lapse analysis was performed on two 3D seismic datasets (4D seismic) acquired over the northern part of the Gullfaks oilfield in the North Sea. The first survey was acquired in 1985, prior to the production start in 1986, a second survey was acquired in 1995, followed by a third in 1996. The drainage interpretations carried out by means of the repeated seismic datasets are in reasonable agreement with similar interpretations performed the traditional way by reservoir engineers, and have locally resulted in an improved understanding of drainage. The time lapse seismic data are routinely used in well planning projects on Gullfaks. In 1997, two wells were drilled into segments that, based on repeated seismic data, were interpreted to be undrained. Both wells encountered oil-filled reservoirs. Although these wells were not based on time lapse seismic alone, the wells served as a good test of the reservoir management potential associated with the 4D technology. Furthermore, waterfront movements predicted on the basis of time lapse seismic data are confirmed by well observations.